diff --git a/lib/jasmine-core/jasmine-html.js b/lib/jasmine-core/jasmine-html.js
index 8c098d84..9e504590 100644
--- a/lib/jasmine-core/jasmine-html.js
+++ b/lib/jasmine-core/jasmine-html.js
@@ -50,10 +50,16 @@ const getJasmineHtmlRequireObj = (function() {
     private$.FailuresView = htmlRequire.FailuresView(j$, private$);
     private$.PerformanceView = htmlRequire.PerformanceView(j$, private$);
     private$.TabBar = htmlRequire.TabBar(j$, private$);
-    j$.HtmlReporterV2Urls = htmlRequire.HtmlReporterV2Urls(j$, private$);
-    j$.HtmlReporterV2 = htmlRequire.HtmlReporterV2(j$, private$);
-    j$.QueryString = htmlRequire.QueryString();
     private$.HtmlSpecFilterV2 = htmlRequire.HtmlSpecFilterV2();
+
+    for (const k of ['HtmlReporterV2Urls', 'HtmlReporterV2', 'QueryString']) {
+      Object.defineProperty(j$, k, {
+        enumerable: true,
+        configurable: false,
+        writable: false,
+        value: htmlRequire[k](j$, private$)
+      });
+    }
   };
 
   return getJasmineHtmlRequire;
@@ -101,6 +107,7 @@ getJasmineHtmlRequireObj().QueryString = function() {
      */
     constructor(options) {
       this.#getWindowLocation = options.getWindowLocation;
+      Object.freeze(this);
     }
 
     /**
@@ -168,6 +175,7 @@ getJasmineHtmlRequireObj().QueryString = function() {
     return '?' + qStrPairs.join('&');
   }
 
+  Object.freeze(QueryString.prototype);
   return QueryString;
 };
 
@@ -772,6 +780,8 @@ getJasmineHtmlRequireObj().HtmlReporterV2 = function(j$, private$) {
       );
       this.#container.appendChild(this.#htmlReporterMain);
       this.#failures.show();
+
+      Object.freeze(this);
     }
 
     jasmineStarted(options) {
@@ -949,6 +959,7 @@ getJasmineHtmlRequireObj().HtmlReporterV2 = function(j$, private$) {
     }
   }
 
+  Object.freeze(HtmlReporterV2.prototype);
   return HtmlReporterV2;
 };
 
@@ -972,6 +983,8 @@ getJasmineHtmlRequireObj().HtmlReporterV2Urls = function(j$, private$) {
             return window.location;
           }
         });
+
+      Object.freeze(this);
     }
 
     /**
@@ -1022,6 +1035,7 @@ getJasmineHtmlRequireObj().HtmlReporterV2Urls = function(j$, private$) {
     }
   }
 
+  Object.freeze(HtmlReporterV2Urls.prototype);
   return HtmlReporterV2Urls;
 };
 
diff --git a/lib/jasmine-core/jasmine.js b/lib/jasmine-core/jasmine.js
index 95b3d2d7..b9612e63 100644
--- a/lib/jasmine-core/jasmine.js
+++ b/lib/jasmine-core/jasmine.js
@@ -141,6 +141,28 @@ const getJasmineRequireObj = (function() {
 
     private$.loadedAsBrowserEsm = loadedAsBrowserEsm;
 
+    // Prevent monkey patching of existing properties but allow adding new ones.
+    // jasmine-html.js needs to be able to add to the jasmine namespace.
+    // jasmine-ajax also installs itself this way.
+    const writeable = [
+      'DEFAULT_TIMEOUT_INTERVAL',
+      'MAX_PRETTY_PRINT_ARRAY_LENGTH',
+      'MAX_PRETTY_PRINT_CHARS',
+      'MAX_PRETTY_PRINT_DEPTH'
+    ];
+    const descriptors = Object.getOwnPropertyDescriptors(j$);
+
+    for (const [k, d] of Object.entries(descriptors)) {
+      if (!writeable.includes(k)) {
+        Object.defineProperty(j$, k, {
+          value: d.value,
+          enumerable: d.enumerable,
+          configurable: false,
+          writable: false
+        });
+      }
+    }
+
     return { jasmine: j$, private: private$ };
   };
 
@@ -241,6 +263,7 @@ getJasmineRequireObj().base = function(j$, private$, jasmineGlobal) {
    */
   let DEFAULT_TIMEOUT_INTERVAL = 5000;
   Object.defineProperty(j$, 'DEFAULT_TIMEOUT_INTERVAL', {
+    enumerable: true,
     get: function() {
       return DEFAULT_TIMEOUT_INTERVAL;
     },
@@ -2077,6 +2100,8 @@ getJasmineRequireObj().Env = function(j$, private$) {
     this.cleanup_ = function() {
       uninstallGlobalErrors();
     };
+
+    Object.freeze(this);
   }
 
   function indirectCallerFilename(depth) {
@@ -2087,6 +2112,8 @@ getJasmineRequireObj().Env = function(j$, private$) {
     return frames[depth] && frames[depth].file;
   }
 
+  Object.freeze(Env);
+  Object.freeze(Env.prototype);
   return Env;
 };
 
@@ -3009,6 +3036,8 @@ callbacks to execute _before_ running the next one.
     setInterval[IsMockClockTimingFn] = true;
     clearInterval[IsMockClockTimingFn] = true;
 
+    Object.freeze(this);
+
     return this;
 
     // Advances the Clock's time until the mode changes.
@@ -3162,6 +3191,8 @@ callbacks to execute _before_ running the next one.
   };
 
   Clock.IsMockClockTimingFn = IsMockClockTimingFn;
+  Object.freeze(Clock);
+  Object.freeze(Clock.prototype);
   return Clock;
 };
 
@@ -8038,9 +8069,12 @@ getJasmineRequireObj().ParallelReportDispatcher = function(j$, private$) {
       for (const eventName of private$.reporterEvents) {
         this[eventName] = dispatcher[eventName].bind(dispatcher);
       }
+
+      Object.freeze(this);
     }
   }
 
+  Object.freeze(ParallelReportDispatcher.prototype);
   return ParallelReportDispatcher;
 };
 
@@ -11698,8 +11732,12 @@ getJasmineRequireObj().Timer = function() {
     this.elapsed = function() {
       return now() - startTime;
     };
+
+    Object.freeze(this);
   }
 
+  Object.freeze(Timer);
+  Object.freeze(Timer.prototype);
   return Timer;
 };
 
diff --git a/spec/core/ClockSpec.js b/spec/core/ClockSpec.js
index a4d92f85..7a5ad19f 100644
--- a/spec/core/ClockSpec.js
+++ b/spec/core/ClockSpec.js
@@ -1247,4 +1247,8 @@ describe('Clock (acceptance)', function() {
 
     clock.tick(400);
   });
+
+  isNonMonkeyPatchableClass(privateUnderTest.Clock, function() {
+    return new privateUnderTest.Clock({}, function() {}, {});
+  });
 });
diff --git a/spec/core/EnvSpec.js b/spec/core/EnvSpec.js
index 3da94a51..ec6c9a66 100644
--- a/spec/core/EnvSpec.js
+++ b/spec/core/EnvSpec.js
@@ -874,4 +874,8 @@ describe('Env', function() {
       }).toThrowError('Jasmine cannot be configured via Env in parallel mode');
     });
   });
+
+  isNonMonkeyPatchableClass(privateUnderTest.Env, function() {
+    return new privateUnderTest.Env();
+  });
 });
diff --git a/spec/core/ParallelReportDispatcherSpec.js b/spec/core/ParallelReportDispatcherSpec.js
index b4d2e137..e7eaf352 100644
--- a/spec/core/ParallelReportDispatcherSpec.js
+++ b/spec/core/ParallelReportDispatcherSpec.js
@@ -160,6 +160,13 @@ describe('ParallelReportDispatcher', function() {
     );
   });
 
+  isNonMonkeyPatchableClass(
+    jasmineUnderTest.ParallelReportDispatcher,
+    function() {
+      return new jasmineUnderTest.ParallelReportDispatcher();
+    }
+  );
+
   function mockGlobalErrors() {
     const globalErrors = jasmine.createSpyObj('globalErrors', [
       'install',
diff --git a/spec/core/TimerSpec.js b/spec/core/TimerSpec.js
index 7d23d10a..1944eec5 100644
--- a/spec/core/TimerSpec.js
+++ b/spec/core/TimerSpec.js
@@ -30,4 +30,8 @@ describe('Timer', function() {
       expect(timer.elapsed()).toEqual(jasmine.any(Number));
     });
   });
+
+  isNonMonkeyPatchableClass(jasmineUnderTest.Timer, function() {
+    return new jasmineUnderTest.Timer();
+  });
 });
diff --git a/spec/core/jasmineNamespaceSpec.js b/spec/core/jasmineNamespaceSpec.js
index 93ae2c3b..bec7311b 100644
--- a/spec/core/jasmineNamespaceSpec.js
+++ b/spec/core/jasmineNamespaceSpec.js
@@ -13,13 +13,61 @@ describe('The jasmine namespace', function() {
     expect(setDifference(actualKeys, expectedKeys())).toEqual(new Set());
   });
 
+  describe('Preventing monkey patching', function() {
+    const mutable = mutableKeys();
+
+    for (const key of expectedKeys()) {
+      if (mutable.includes(key)) {
+        it(`allows overwriting of jasmine.${key}`, function() {
+          const existingVal = jasmineUnderTest[key];
+
+          try {
+            jasmineUnderTest[key] = 'new value';
+            expect(jasmineUnderTest[key]).toEqual('new value');
+          } finally {
+            jasmineUnderTest[key] = existingVal;
+          }
+        });
+      } else {
+        it(`prevents overwriting of jasmine.${key}`, function() {
+          const existingVal = jasmineUnderTest[key];
+
+          try {
+            jasmineUnderTest[key] = 'monkey patch';
+            expect(jasmineUnderTest[key]).toBe(existingVal);
+          } finally {
+            // This will be a no-op if the test passed, but will prevent state
+            // leakage if it failed.
+            jasmineUnderTest[key] = existingVal;
+          }
+        });
+      }
+    }
+
+    it('allows additions', function() {
+      try {
+        jasmineUnderTest.Ajax = 'it worked';
+        expect(jasmineUnderTest.Ajax).toEqual('it worked');
+      } finally {
+        delete jasmineUnderTest.Ajax;
+      }
+    });
+  });
+
+  function mutableKeys() {
+    return [
+      'MAX_PRETTY_PRINT_ARRAY_LENGTH',
+      'MAX_PRETTY_PRINT_CHARS',
+      'MAX_PRETTY_PRINT_DEPTH',
+      'DEFAULT_TIMEOUT_INTERVAL'
+    ];
+  }
+
   function expectedKeys() {
     // Does not include properties added by requireInterface(), since that isn't
     // called by defineJasmineUnderTest.js/nodeDefineJasmineUnderTest.js.
     const result = new Set([
-      'MAX_PRETTY_PRINT_ARRAY_LENGTH',
-      'MAX_PRETTY_PRINT_CHARS',
-      'MAX_PRETTY_PRINT_DEPTH',
+      ...mutableKeys(),
       'debugLog',
       'getEnv',
       'isSpy',
diff --git a/spec/helpers/monkeyPatchingSpecs.js b/spec/helpers/monkeyPatchingSpecs.js
new file mode 100644
index 00000000..af06f8cf
--- /dev/null
+++ b/spec/helpers/monkeyPatchingSpecs.js
@@ -0,0 +1,67 @@
+globalThis.isNonMonkeyPatchableClass = function(ctor, makeInstance) {
+  describe('Monkey patching prevention', function() {
+    it(`prevents overwriting ${ctor.name}.prototype`, function() {
+      const existing = ctor.prototype;
+
+      try {
+        ctor.prototype = {};
+        expect(ctor.prototype).toBe(existing);
+      } finally {
+        // This will be a no-op if the test passed, but will prevent state
+        // leakage if it failed.
+        ctor.prototype = existing;
+      }
+    });
+
+    it("prevents overwriting an instance's prototype", function() {
+      const instance = makeInstance();
+      let thrown;
+
+      // The message varies from browser to browser, so we can't rely on it
+      try {
+        instance.__proto__ = {};
+      } catch (e) {
+        thrown = e;
+      }
+
+      expect(thrown).toBeInstanceOf(TypeError);
+    });
+
+    it('prevents overwriting prototype properties', function() {
+      let any = false;
+
+      for (const k of Object.getOwnPropertyNames(ctor.prototype)) {
+        any = true;
+        const existingValue = ctor.prototype[k];
+
+        try {
+          ctor.prototype[k] = {};
+          expect(ctor.prototype[k])
+            .withContext(k)
+            .toBe(existingValue);
+        } finally {
+          // This will be a no-op if the test passed, but will prevent state
+          // leakage if it failed.
+          ctor.prototype[k] = existingValue;
+        }
+      }
+
+      expect(any).toBe(true);
+    });
+
+    it('prevents overriding prototype properties', function() {
+      const instance = makeInstance();
+      let any = false;
+
+      for (const k of Object.getOwnPropertyNames(ctor.prototype)) {
+        any = true;
+        instance[k] = {};
+        expect(instance[k])
+          .withContext(k)
+          .toBe(ctor.prototype[k]);
+      }
+
+      expect(any).toBe(true);
+    });
+  });
+};
diff --git a/spec/html/HtmlReporterV2Spec.js b/spec/html/HtmlReporterV2Spec.js
index dcc583a0..d4167f87 100644
--- a/spec/html/HtmlReporterV2Spec.js
+++ b/spec/html/HtmlReporterV2Spec.js
@@ -1396,4 +1396,6 @@ describe('HtmlReporterV2', function() {
       });
     });
   });
+
+  isNonMonkeyPatchableClass(jasmineUnderTest.HtmlReporterV2, setup);
 });
diff --git a/spec/html/HtmlReporterV2UrlsSpec.js b/spec/html/HtmlReporterV2UrlsSpec.js
index e2b99900..64c78436 100644
--- a/spec/html/HtmlReporterV2UrlsSpec.js
+++ b/spec/html/HtmlReporterV2UrlsSpec.js
@@ -63,4 +63,8 @@ describe('HtmlReporterV2Urls', function() {
       return qs;
     }
   });
+
+  isNonMonkeyPatchableClass(jasmineUnderTest.HtmlReporterV2Urls, function() {
+    return new jasmineUnderTest.HtmlReporterV2Urls({});
+  });
 });
diff --git a/spec/html/QueryStringSpec.js b/spec/html/QueryStringSpec.js
index bc8d1770..00ca656a 100644
--- a/spec/html/QueryStringSpec.js
+++ b/spec/html/QueryStringSpec.js
@@ -77,4 +77,12 @@ describe('QueryString', function() {
       expect(queryString.getParam('baz')).toBeFalsy();
     });
   });
+
+  isNonMonkeyPatchableClass(jasmineUnderTest.QueryString, function() {
+    return new jasmineUnderTest.QueryString({
+      getWindowLocation: function() {
+        return { search: '' };
+      }
+    });
+  });
 });
diff --git a/spec/support/jasmine-browser.js b/spec/support/jasmine-browser.js
index e19d9f1a..0771e7fc 100644
--- a/spec/support/jasmine-browser.js
+++ b/spec/support/jasmine-browser.js
@@ -25,6 +25,7 @@ module.exports = {
     'helpers/domHelpers.js',
     'helpers/integrationMatchers.js',
     'helpers/callerFilenameShim.js',
+    'helpers/monkeyPatchingSpecs.js',
     'helpers/defineJasmineUnderTest.js',
     'helpers/resetEnv.js'
   ],
diff --git a/spec/support/jasmine.json b/spec/support/jasmine.json
index e9094d74..3be76276 100644
--- a/spec/support/jasmine.json
+++ b/spec/support/jasmine.json
@@ -10,6 +10,7 @@
     "helpers/integrationMatchers.js",
     "helpers/callerFilenameShim.js",
     "helpers/overrideConsoleLogForCircleCi.js",
+    "helpers/monkeyPatchingSpecs.js",
     "helpers/nodeDefineJasmineUnderTest.js",
     "helpers/resetEnv.js"
   ],
diff --git a/src/core/Clock.js b/src/core/Clock.js
index 883e84df..907fbad6 100644
--- a/src/core/Clock.js
+++ b/src/core/Clock.js
@@ -192,6 +192,8 @@ callbacks to execute _before_ running the next one.
     setInterval[IsMockClockTimingFn] = true;
     clearInterval[IsMockClockTimingFn] = true;
 
+    Object.freeze(this);
+
     return this;
 
     // Advances the Clock's time until the mode changes.
@@ -345,5 +347,7 @@ callbacks to execute _before_ running the next one.
   };
 
   Clock.IsMockClockTimingFn = IsMockClockTimingFn;
+  Object.freeze(Clock);
+  Object.freeze(Clock.prototype);
   return Clock;
 };
diff --git a/src/core/Env.js b/src/core/Env.js
index 6e66a76d..d44d6e7c 100644
--- a/src/core/Env.js
+++ b/src/core/Env.js
@@ -835,6 +835,8 @@ getJasmineRequireObj().Env = function(j$, private$) {
     this.cleanup_ = function() {
       uninstallGlobalErrors();
     };
+
+    Object.freeze(this);
   }
 
   function indirectCallerFilename(depth) {
@@ -845,5 +847,7 @@ getJasmineRequireObj().Env = function(j$, private$) {
     return frames[depth] && frames[depth].file;
   }
 
+  Object.freeze(Env);
+  Object.freeze(Env.prototype);
   return Env;
 };
diff --git a/src/core/ParallelReportDispatcher.js b/src/core/ParallelReportDispatcher.js
index 360478d6..72d29c8c 100644
--- a/src/core/ParallelReportDispatcher.js
+++ b/src/core/ParallelReportDispatcher.js
@@ -87,8 +87,11 @@ getJasmineRequireObj().ParallelReportDispatcher = function(j$, private$) {
       for (const eventName of private$.reporterEvents) {
         this[eventName] = dispatcher[eventName].bind(dispatcher);
       }
+
+      Object.freeze(this);
     }
   }
 
+  Object.freeze(ParallelReportDispatcher.prototype);
   return ParallelReportDispatcher;
 };
diff --git a/src/core/Timer.js b/src/core/Timer.js
index a65064ba..b1726393 100644
--- a/src/core/Timer.js
+++ b/src/core/Timer.js
@@ -39,7 +39,11 @@ getJasmineRequireObj().Timer = function() {
     this.elapsed = function() {
       return now() - startTime;
     };
+
+    Object.freeze(this);
   }
 
+  Object.freeze(Timer);
+  Object.freeze(Timer.prototype);
   return Timer;
 };
diff --git a/src/core/base.js b/src/core/base.js
index 9f1fa540..523bf1c2 100644
--- a/src/core/base.js
+++ b/src/core/base.js
@@ -43,6 +43,7 @@ getJasmineRequireObj().base = function(j$, private$, jasmineGlobal) {
    */
   let DEFAULT_TIMEOUT_INTERVAL = 5000;
   Object.defineProperty(j$, 'DEFAULT_TIMEOUT_INTERVAL', {
+    enumerable: true,
     get: function() {
       return DEFAULT_TIMEOUT_INTERVAL;
     },
diff --git a/src/core/requireCore.js b/src/core/requireCore.js
index 1d153dc8..dd9b5cc4 100644
--- a/src/core/requireCore.js
+++ b/src/core/requireCore.js
@@ -116,6 +116,28 @@ const getJasmineRequireObj = (function() {
 
     private$.loadedAsBrowserEsm = loadedAsBrowserEsm;
 
+    // Prevent monkey patching of existing properties but allow adding new ones.
+    // jasmine-html.js needs to be able to add to the jasmine namespace.
+    // jasmine-ajax also installs itself this way.
+    const writeable = [
+      'DEFAULT_TIMEOUT_INTERVAL',
+      'MAX_PRETTY_PRINT_ARRAY_LENGTH',
+      'MAX_PRETTY_PRINT_CHARS',
+      'MAX_PRETTY_PRINT_DEPTH'
+    ];
+    const descriptors = Object.getOwnPropertyDescriptors(j$);
+
+    for (const [k, d] of Object.entries(descriptors)) {
+      if (!writeable.includes(k)) {
+        Object.defineProperty(j$, k, {
+          value: d.value,
+          enumerable: d.enumerable,
+          configurable: false,
+          writable: false
+        });
+      }
+    }
+
     return { jasmine: j$, private: private$ };
   };
 
diff --git a/src/html/HtmlReporterV2.js b/src/html/HtmlReporterV2.js
index c0a0515d..5fa038e0 100644
--- a/src/html/HtmlReporterV2.js
+++ b/src/html/HtmlReporterV2.js
@@ -98,6 +98,8 @@ getJasmineHtmlRequireObj().HtmlReporterV2 = function(j$, private$) {
       );
       this.#container.appendChild(this.#htmlReporterMain);
       this.#failures.show();
+
+      Object.freeze(this);
     }
 
     jasmineStarted(options) {
@@ -275,5 +277,6 @@ getJasmineHtmlRequireObj().HtmlReporterV2 = function(j$, private$) {
     }
   }
 
+  Object.freeze(HtmlReporterV2.prototype);
   return HtmlReporterV2;
 };
diff --git a/src/html/HtmlReporterV2Urls.js b/src/html/HtmlReporterV2Urls.js
index c6ef4825..41271170 100644
--- a/src/html/HtmlReporterV2Urls.js
+++ b/src/html/HtmlReporterV2Urls.js
@@ -18,6 +18,8 @@ getJasmineHtmlRequireObj().HtmlReporterV2Urls = function(j$, private$) {
             return window.location;
           }
         });
+
+      Object.freeze(this);
     }
 
     /**
@@ -68,5 +70,6 @@ getJasmineHtmlRequireObj().HtmlReporterV2Urls = function(j$, private$) {
     }
   }
 
+  Object.freeze(HtmlReporterV2Urls.prototype);
   return HtmlReporterV2Urls;
 };
diff --git a/src/html/QueryString.js b/src/html/QueryString.js
index 9946c19a..7db5ba37 100644
--- a/src/html/QueryString.js
+++ b/src/html/QueryString.js
@@ -14,6 +14,7 @@ getJasmineHtmlRequireObj().QueryString = function() {
      */
     constructor(options) {
       this.#getWindowLocation = options.getWindowLocation;
+      Object.freeze(this);
     }
 
     /**
@@ -81,5 +82,6 @@ getJasmineHtmlRequireObj().QueryString = function() {
     return '?' + qStrPairs.join('&');
   }
 
+  Object.freeze(QueryString.prototype);
   return QueryString;
 };
diff --git a/src/html/requireHtml.js b/src/html/requireHtml.js
index 13ee40a2..fe99507c 100644
--- a/src/html/requireHtml.js
+++ b/src/html/requireHtml.js
@@ -25,10 +25,16 @@ const getJasmineHtmlRequireObj = (function() {
     private$.FailuresView = htmlRequire.FailuresView(j$, private$);
     private$.PerformanceView = htmlRequire.PerformanceView(j$, private$);
     private$.TabBar = htmlRequire.TabBar(j$, private$);
-    j$.HtmlReporterV2Urls = htmlRequire.HtmlReporterV2Urls(j$, private$);
-    j$.HtmlReporterV2 = htmlRequire.HtmlReporterV2(j$, private$);
-    j$.QueryString = htmlRequire.QueryString();
     private$.HtmlSpecFilterV2 = htmlRequire.HtmlSpecFilterV2();
+
+    for (const k of ['HtmlReporterV2Urls', 'HtmlReporterV2', 'QueryString']) {
+      Object.defineProperty(j$, k, {
+        enumerable: true,
+        configurable: false,
+        writable: false,
+        value: htmlRequire[k](j$, private$)
+      });
+    }
   };
 
   return getJasmineHtmlRequire;