diff --git a/remote/webdriver-bidi/Realm.jsm b/remote/webdriver-bidi/Realm.jsm
new file mode 100644
index 0000000000000..76da89ae8220d
--- /dev/null
+++ b/remote/webdriver-bidi/Realm.jsm
@@ -0,0 +1,191 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+var EXPORTED_SYMBOLS = ["WindowRealm"];
+
+const { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+
+const lazy = {};
+XPCOMUtils.defineLazyModuleGetters(lazy, {
+  addDebuggerToGlobal: "resource://gre/modules/jsdebugger.jsm",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "dbg", () => {
+  // eslint-disable-next-line mozilla/reject-globalThis-modification
+  lazy.addDebuggerToGlobal(globalThis);
+  return new Debugger();
+});
+
+/**
+ * @typedef {string} RealmType
+ **/
+
+/**
+ * Enum of realm types.
+ *
+ * @readonly
+ * @enum {RealmType}
+ **/
+const RealmType = {
+  AudioWorklet: "audio-worklet",
+  DedicatedWorker: "dedicated-worker",
+  PaintWorklet: "paint-worklet",
+  ServiceWorker: "service-worker",
+  SharedWorker: "shared-worker",
+  Window: "window",
+  Worker: "worker",
+  Worklet: "worklet",
+};
+
+/**
+ * Base class that wraps any kind of WebDriver BiDi realm.
+ */
+class Realm {
+  #id;
+
+  constructor() {
+    this.#id = Services.uuid
+      .generateUUID()
+      .toString()
+      .slice(1, -1);
+  }
+
+  /**
+   * Get the unique identifier of the realm instance.
+   *
+   * @return {string} The unique identifier.
+   */
+  get id() {
+    return this.#id;
+  }
+}
+
+/**
+ * Wrapper for Window realms including sandbox objects.
+ */
+class WindowRealm extends Realm {
+  #globalObject;
+  #globalObjectReference;
+  #window;
+
+  static type = RealmType.Window;
+
+  /**
+   *
+   * @param {Window} window
+   *     The window global to wrap.
+   * @param {Object} options
+   * @param {string=} options.sandboxName
+   *     Name of the sandbox to create if specified. Defaults to `null`.
+   */
+  constructor(window, options = {}) {
+    const { sandboxName = null } = options;
+
+    super();
+
+    this.#window = window;
+    this.#globalObject =
+      sandboxName === null ? this.#window : this.#createSandbox();
+    this.#globalObjectReference = lazy.dbg.makeGlobalObjectReference(
+      this.#globalObject
+    );
+
+    lazy.dbg.enableAsyncStack(this.#globalObject);
+  }
+
+  destroy() {
+    lazy.dbg.disableAsyncStack(this.#globalObject);
+
+    this.#globalObjectReference = null;
+    this.#globalObject = null;
+    this.#window = null;
+  }
+
+  get globalObjectReference() {
+    return this.#globalObjectReference;
+  }
+
+  #cloneAsDebuggerObject(obj) {
+    // To use an object created in the priviledged Debugger compartment from
+    // the content compartment, we need to first clone it into the target
+    // compartment and then retrieve the corresponding Debugger.Object wrapper.
+    const proxyObject = Cu.cloneInto(
+      obj,
+      this.#globalObjectReference.unsafeDereference()
+    );
+
+    return this.#globalObjectReference.makeDebuggeeValue(proxyObject);
+  }
+
+  #createSandbox() {
+    const win = this.#window;
+    const opts = {
+      sameZoneAs: win,
+      sandboxPrototype: win,
+      wantComponents: false,
+      wantXrays: true,
+    };
+
+    return new Cu.Sandbox(win, opts);
+  }
+
+  /**
+   * Evaluates a provided expression in the context of the current realm.
+   *
+   * @param {string} expression
+   *     The expression to evaluate.
+   *
+   * @return {Object}
+   *     - evaluationStatus {EvaluationStatus} One of "normal", "throw".
+   *     - exceptionDetails {ExceptionDetails=} the details of the exception if
+   *       the evaluation status was "throw".
+   *     - result {RemoteValue=} the result of the evaluation serialized as a
+   *       RemoteValue if the evaluation status was "normal".
+   */
+  executeInGlobal(expression) {
+    return this.#globalObjectReference.executeInGlobal(expression, {
+      url: this.#window.document.baseURI,
+    });
+  }
+
+  /**
+   * Call a function in the context of the current realm.
+   *
+   * @param {string} functionDeclaration
+   *     The body of the function to call.
+   * @param {Array<Object>} functionArguments
+   *     The arguments to pass to the function call.
+   * @param {Object} thisParameter
+   *     The value of the `this` keyword for the function call.
+   *
+   * @return {Object}
+   *     - evaluationStatus {EvaluationStatus} One of "normal", "throw".
+   *     - exceptionDetails {ExceptionDetails=} the details of the exception if
+   *       the evaluation status was "throw".
+   *     - result {RemoteValue=} the result of the evaluation serialized as a
+   *       RemoteValue if the evaluation status was "normal".
+   */
+  executeInGlobalWithBindings(
+    functionDeclaration,
+    functionArguments,
+    thisParameter
+  ) {
+    const expression = `(${functionDeclaration}).apply(__bidi_this, __bidi_args)`;
+
+    return this.#globalObjectReference.executeInGlobalWithBindings(
+      expression,
+      {
+        __bidi_args: this.#cloneAsDebuggerObject(functionArguments),
+        __bidi_this: this.#cloneAsDebuggerObject(thisParameter),
+      },
+      {
+        url: this.#window.document.baseURI,
+      }
+    );
+  }
+}
diff --git a/remote/webdriver-bidi/jar.mn b/remote/webdriver-bidi/jar.mn
index c4bca881075a2..59b1ea7f14fd3 100644
--- a/remote/webdriver-bidi/jar.mn
+++ b/remote/webdriver-bidi/jar.mn
@@ -6,6 +6,7 @@ remote.jar:
 % content remote %content/
 
   content/webdriver-bidi/NewSessionHandler.jsm (NewSessionHandler.jsm)
+  content/webdriver-bidi/Realm.jsm (Realm.jsm)
   content/webdriver-bidi/RemoteValue.jsm (RemoteValue.jsm)
   content/webdriver-bidi/WebDriverBiDi.jsm (WebDriverBiDi.jsm)
   content/webdriver-bidi/WebDriverBiDiConnection.jsm (WebDriverBiDiConnection.jsm)
diff --git a/remote/webdriver-bidi/modules/root/script.jsm b/remote/webdriver-bidi/modules/root/script.jsm
index 57a6237820b70..67681fcef1b09 100644
--- a/remote/webdriver-bidi/modules/root/script.jsm
+++ b/remote/webdriver-bidi/modules/root/script.jsm
@@ -297,7 +297,7 @@ class ScriptModule extends Module {
   }
 
   #buildReturnValue(evaluationResult) {
-    const rv = { realm: evaluationResult.realm.realm };
+    const rv = { realm: evaluationResult.realmId };
     switch (evaluationResult.evaluationStatus) {
       // TODO: Compare with EvaluationStatus.Normal after Bug 1774444 is fixed.
       case "normal":
diff --git a/remote/webdriver-bidi/modules/windowglobal/script.jsm b/remote/webdriver-bidi/modules/windowglobal/script.jsm
index 14ca923f11b2c..c5072ee2137ca 100644
--- a/remote/webdriver-bidi/modules/windowglobal/script.jsm
+++ b/remote/webdriver-bidi/modules/windowglobal/script.jsm
@@ -16,24 +16,17 @@ const { Module } = ChromeUtils.import(
 
 const lazy = {};
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  addDebuggerToGlobal: "resource://gre/modules/jsdebugger.jsm",
-
   deserialize: "chrome://remote/content/webdriver-bidi/RemoteValue.jsm",
   error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   getFramesFromStack: "chrome://remote/content/shared/Stack.jsm",
   isChromeFrame: "chrome://remote/content/shared/Stack.jsm",
   serialize: "chrome://remote/content/webdriver-bidi/RemoteValue.jsm",
   stringify: "chrome://remote/content/webdriver-bidi/RemoteValue.jsm",
-});
-
-XPCOMUtils.defineLazyGetter(lazy, "dbg", () => {
-  // eslint-disable-next-line mozilla/reject-globalThis-modification
-  lazy.addDebuggerToGlobal(globalThis);
-  return new Debugger();
+  WindowRealm: "chrome://remote/content/webdriver-bidi/Realm.jsm",
 });
 
 /**
- * @typedef {Object} EvaluationStatus
+ * @typedef {string} EvaluationStatus
  **/
 
 /**
@@ -47,49 +40,26 @@ const EvaluationStatus = {
   Throw: "throw",
 };
 
-/**
- * @typedef {Object} RealmType
- **/
-
-/**
- * Enum of realm types.
- *
- * @readonly
- * @enum {RealmType}
- **/
-const RealmType = {
-  AudioWorklet: "audio-worklet",
-  DedicatedWorker: "dedicated-worker",
-  PaintWorklet: "paint-worklet",
-  ServiceWorker: "service-worker",
-  SharedWorker: "shared-worker",
-  Window: "window",
-  Worker: "worker",
-  Worklet: "worklet",
-};
-
-const WINDOW_GLOBAL_KEY = Symbol();
-
 class ScriptModule extends Module {
-  #globals;
-  #realmIds;
+  #defaultRealm;
+  #realms;
 
   constructor(messageHandler) {
     super(messageHandler);
 
-    // Maps sandbox names as keys to sandboxes as values and
-    // WINDOW_GLOBAL_KEY key to default window-global
-    this.#globals = new Map();
-    // Maps default window-global and sandboxes as keys to realmIds as values
-    this.#realmIds = new WeakMap();
+    this.#defaultRealm = new lazy.WindowRealm(this.messageHandler.window);
+
+    // Maps sandbox names to instances of window realms.
+    this.#realms = new Map();
   }
 
   destroy() {
-    for (const global of this.#globals.values()) {
-      lazy.dbg.disableAsyncStack(global.unsafeDereference());
+    this.#defaultRealm.destroy();
+
+    for (const realm of this.#realms.values()) {
+      realm.destroy();
     }
-    this.#globals = null;
-    this.#realmIds = null;
+    this.#realms = null;
   }
 
   #buildExceptionDetails(exception, stack) {
@@ -119,9 +89,9 @@ class ScriptModule extends Module {
     };
   }
 
-  async #buildReturnValue(rv, awaitPromise, globalObjectReference) {
+  async #buildReturnValue(rv, realm, awaitPromise) {
     let evaluationStatus, exception, result, stack;
-    const realm = this.#getRealm(globalObjectReference);
+
     if ("return" in rv) {
       evaluationStatus = EvaluationStatus.Normal;
       if (
@@ -134,10 +104,12 @@ class ScriptModule extends Module {
           // Force wrapping the promise resolution result in a Debugger.Object
           // wrapper for consistency with the synchronous codepath.
           const asyncResult = await rv.return.unsafeDereference();
-          result = globalObjectReference.makeDebuggeeValue(asyncResult);
+          result = realm.globalObjectReference.makeDebuggeeValue(asyncResult);
         } catch (asyncException) {
           evaluationStatus = EvaluationStatus.Throw;
-          exception = globalObjectReference.makeDebuggeeValue(asyncException);
+          exception = realm.globalObjectReference.makeDebuggeeValue(
+            asyncException
+          );
           stack = rv.return.promiseResolutionSite;
         }
       } else {
@@ -157,13 +129,13 @@ class ScriptModule extends Module {
         return {
           evaluationStatus,
           result: lazy.serialize(this.#toRawObject(result), 1),
-          realm,
+          realmId: realm.id,
         };
       case EvaluationStatus.Throw:
         return {
           evaluationStatus,
           exceptionDetails: this.#buildExceptionDetails(exception, stack),
-          realm,
+          realmId: realm.id,
         };
       default:
         throw new lazy.error.UnsupportedOperationError(
@@ -172,67 +144,22 @@ class ScriptModule extends Module {
     }
   }
 
-  #buildRealmId(sandboxName) {
-    let realmId = String(this.messageHandler.innerWindowId);
-    if (sandboxName) {
-      realmId += "-sandbox-" + sandboxName;
+  #getRealmFromSandboxName(sandboxName) {
+    if (sandboxName === null) {
+      return this.#defaultRealm;
     }
-    return realmId;
-  }
-
-  #cloneAsDebuggerObject(obj, globalObjectReference) {
-    // To use an object created in the priviledged Debugger compartment from
-    // the content compartment, we need to first clone it into the target
-    // compartment and then retrieve the corresponding Debugger.Object wrapper.
-    const proxyObject = Cu.cloneInto(
-      obj,
-      globalObjectReference.unsafeDereference()
-    );
-    return globalObjectReference.makeDebuggeeValue(proxyObject);
-  }
 
-  #createSandbox() {
-    const win = this.messageHandler.window;
-    const opts = {
-      sameZoneAs: win,
-      sandboxPrototype: win,
-      wantComponents: false,
-      wantXrays: true,
-    };
-
-    return new Cu.Sandbox(win, opts);
-  }
-
-  #getGlobalObjectReference(sandboxName) {
-    const key = sandboxName !== null ? sandboxName : WINDOW_GLOBAL_KEY;
-    if (this.#globals.has(key)) {
-      return this.#globals.get(key);
+    if (this.#realms.has(sandboxName)) {
+      return this.#realms.get(sandboxName);
     }
 
-    const globalObject =
-      key == WINDOW_GLOBAL_KEY
-        ? this.messageHandler.window
-        : this.#createSandbox();
-
-    const globalObjectReference = lazy.dbg.makeGlobalObjectReference(
-      globalObject
-    );
-    lazy.dbg.enableAsyncStack(globalObject);
-
-    this.#globals.set(key, globalObjectReference);
-    this.#realmIds.set(globalObjectReference, this.#buildRealmId(sandboxName));
+    const realm = new lazy.WindowRealm(this.messageHandler.window, {
+      sandboxName,
+    });
 
-    return globalObjectReference;
-  }
+    this.#realms.set(sandboxName, realm);
 
-  #getRealm(globalObjectReference) {
-    // TODO: Return an actual realm once we have proper realm support.
-    // See Bug 1766240.
-    return {
-      realm: this.#realmIds.get(globalObjectReference),
-      origin: null,
-      type: RealmType.Window,
-    };
+    return realm;
   }
 
   #toRawObject(maybeDebuggerObject) {
@@ -287,34 +214,22 @@ class ScriptModule extends Module {
     } = options;
 
     const deserializedArguments =
-      commandArguments != null
-        ? commandArguments.map(a => lazy.deserialize(a))
+      commandArguments !== null
+        ? commandArguments.map(arg => lazy.deserialize(arg))
         : [];
 
     const deserializedThis =
-      thisParameter != null ? lazy.deserialize(thisParameter) : null;
-
-    const expression = `(${functionDeclaration}).apply(__bidi_this, __bidi_args)`;
-
-    const globalObjectReference = this.#getGlobalObjectReference(sandboxName);
-    const rv = globalObjectReference.executeInGlobalWithBindings(
-      expression,
-      {
-        __bidi_args: this.#cloneAsDebuggerObject(
-          deserializedArguments,
-          globalObjectReference
-        ),
-        __bidi_this: this.#cloneAsDebuggerObject(
-          deserializedThis,
-          globalObjectReference
-        ),
-      },
-      {
-        url: this.messageHandler.window.document.baseURI,
-      }
+      thisParameter !== null ? lazy.deserialize(thisParameter) : null;
+
+    const realm = this.#getRealmFromSandboxName(sandboxName);
+
+    const rv = realm.executeInGlobalWithBindings(
+      functionDeclaration,
+      deserializedArguments,
+      deserializedThis
     );
 
-    return this.#buildReturnValue(rv, awaitPromise, globalObjectReference);
+    return this.#buildReturnValue(rv, realm, awaitPromise);
   }
 
   /**
@@ -339,12 +254,10 @@ class ScriptModule extends Module {
   async evaluateExpression(options) {
     const { awaitPromise, expression, sandbox: sandboxName = null } = options;
 
-    const globalObjectReference = this.#getGlobalObjectReference(sandboxName);
-    const rv = globalObjectReference.executeInGlobal(expression, {
-      url: this.messageHandler.window.document.baseURI,
-    });
+    const realm = this.#getRealmFromSandboxName(sandboxName);
+    const rv = realm.executeInGlobal(expression);
 
-    return this.#buildReturnValue(rv, awaitPromise, globalObjectReference);
+    return this.#buildReturnValue(rv, realm, awaitPromise);
   }
 }