diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index 91969d27f35b..336e7b9a0585 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -19626,6 +19626,11 @@ bool Document::HasStorageAccessPermissionGrantedByAllowList() {
          nsILoadInfo::StoragePermissionAllowListed;
 }
 
+bool Document::InAndroidPipMode() const {
+  auto* bc = BrowserChild::GetFrom(GetDocShell());
+  return bc && bc->InAndroidPipMode();
+}
+
 nsIPrincipal* Document::EffectiveStoragePrincipal() const {
   if (!StaticPrefs::
           privacy_partition_always_partition_third_party_non_cookie_storage()) {
diff --git a/dom/base/Document.h b/dom/base/Document.h
index 4d696b6906ce..b41d8be0740c 100644
--- a/dom/base/Document.h
+++ b/dom/base/Document.h
@@ -2192,6 +2192,10 @@ class Document : public nsINode,
    * Returns true if this document was created from a nsXULPrototypeDocument.
    */
   bool LoadedFromPrototype() const { return mPrototypeDocument; }
+
+  /* Returns true if we're currently in Android's PiP mode. */
+  bool InAndroidPipMode() const;
+
   /**
    * Returns the prototype the document was created from, or null if it was not
    * created from a prototype.
diff --git a/dom/ipc/BrowserChild.cpp b/dom/ipc/BrowserChild.cpp
index 24b950350a1f..66dfe24b5b4d 100644
--- a/dom/ipc/BrowserChild.cpp
+++ b/dom/ipc/BrowserChild.cpp
@@ -285,6 +285,7 @@ BrowserChild::BrowserChild(ContentChild* aManager, const TabId& aTabId,
       mTriedBrowserInit(false),
       mHasValidInnerSize(false),
       mDestroyed(false),
+      mInAndroidPipMode(false),
       mIsTopLevel(aIsTopLevel),
       mIsTransparent(false),
       mIPCOpen(false),
@@ -1257,6 +1258,21 @@ mozilla::ipc::IPCResult BrowserChild::RecvKeyboardHeightChanged(
   return IPC_OK();
 }
 
+mozilla::ipc::IPCResult BrowserChild::RecvAndroidPipModeChanged(bool aPipMode) {
+  if (mInAndroidPipMode == aPipMode) {
+    return IPC_OK();
+  }
+  mInAndroidPipMode = aPipMode;
+  if (RefPtr<Document> document = GetTopLevelDocument()) {
+    nsContentUtils::DispatchEventOnlyToChrome(
+        document, document,
+        aPipMode ? u"MozAndroidPipModeEntered"_ns
+                 : u"MozAndroidPipModeExited"_ns,
+        CanBubble::eYes, Cancelable::eNo, /* DefaultAction */ nullptr);
+  }
+  return IPC_OK();
+}
+
 mozilla::ipc::IPCResult BrowserChild::RecvSuppressDisplayport(
     const bool& aEnabled) {
   if (RefPtr<PresShell> presShell = GetTopLevelPresShell()) {
diff --git a/dom/ipc/BrowserChild.h b/dom/ipc/BrowserChild.h
index 27767514c1d8..c41e13086802 100644
--- a/dom/ipc/BrowserChild.h
+++ b/dom/ipc/BrowserChild.h
@@ -85,12 +85,8 @@ class SessionStoreChild;
 class RequestData;
 class WebProgressData;
 
-#define DOM_BROWSERCHILD_IID                         \
-  {                                                  \
-    0x58a5775d, 0xba05, 0x45bf, {                    \
-      0xbd, 0xb8, 0xd7, 0x61, 0xf9, 0x01, 0x01, 0x31 \
-    }                                                \
-  }
+#define DOM_BROWSERCHILD_IID \
+  {0x58a5775d, 0xba05, 0x45bf, {0xbd, 0xb8, 0xd7, 0x61, 0xf9, 0x01, 0x01, 0x31}}
 
 class BrowserChildMessageManager : public ContentFrameMessageManager,
                                    public nsIMessageSender,
@@ -287,6 +283,8 @@ class BrowserChild final : public nsMessageManagerScriptExecutor,
   mozilla::ipc::IPCResult RecvKeyboardHeightChanged(
       const mozilla::ScreenIntCoord& aHeight);
 
+  mozilla::ipc::IPCResult RecvAndroidPipModeChanged(bool aPipMode);
+
   mozilla::ipc::IPCResult RecvActivate(uint64_t aActionId);
 
   mozilla::ipc::IPCResult RecvDeactivate(uint64_t aActionId);
@@ -563,6 +561,8 @@ class BrowserChild final : public nsMessageManagerScriptExecutor,
   };
   mozilla::ScreenIntCoord GetKeyboardHeight() const { return mKeyboardHeight; }
 
+  bool InAndroidPipMode() const { return mInAndroidPipMode; }
+
   bool IPCOpen() const { return mIPCOpen; }
 
   const mozilla::layers::CompositorOptions& GetCompositorOptions() const;
@@ -848,6 +848,8 @@ class BrowserChild final : public nsMessageManagerScriptExecutor,
   bool mTriedBrowserInit : 1;
   bool mHasValidInnerSize : 1;
   bool mDestroyed : 1;
+  // Whether we're in Android's PiP mode.
+  bool mInAndroidPipMode : 1;
 
   // Whether or not this browser is the child part of the top level PBrowser
   // actor in a remote browser.
diff --git a/dom/ipc/BrowserParent.cpp b/dom/ipc/BrowserParent.cpp
index 25137bea3b6e..06f524e60d9c 100644
--- a/dom/ipc/BrowserParent.cpp
+++ b/dom/ipc/BrowserParent.cpp
@@ -1189,7 +1189,7 @@ void BrowserParent::SizeModeChanged(const nsSizeMode& aSizeMode) {
   }
 }
 
-#if defined(MOZ_WIDGET_ANDROID)
+#ifdef MOZ_WIDGET_ANDROID
 void BrowserParent::DynamicToolbarMaxHeightChanged(ScreenIntCoord aHeight) {
   if (!mIsDestroyed) {
     Unused << SendDynamicToolbarMaxHeightChanged(aHeight);
@@ -1207,6 +1207,12 @@ void BrowserParent::KeyboardHeightChanged(ScreenIntCoord aHeight) {
     Unused << SendKeyboardHeightChanged(aHeight);
   }
 }
+
+void BrowserParent::AndroidPipModeChanged(bool aPipMode) {
+  if (!mIsDestroyed) {
+    Unused << SendAndroidPipModeChanged(aPipMode);
+  }
+}
 #endif
 
 void BrowserParent::HandleAccessKey(const WidgetKeyboardEvent& aEvent,
diff --git a/dom/ipc/BrowserParent.h b/dom/ipc/BrowserParent.h
index 501efc92c726..3ec5cc6ea79c 100644
--- a/dom/ipc/BrowserParent.h
+++ b/dom/ipc/BrowserParent.h
@@ -486,10 +486,11 @@ class BrowserParent final : public PBrowserParent,
   void HandleAccessKey(const WidgetKeyboardEvent& aEvent,
                        nsTArray<uint32_t>& aCharCodes);
 
-#if defined(MOZ_WIDGET_ANDROID)
+#ifdef MOZ_WIDGET_ANDROID
   void DynamicToolbarMaxHeightChanged(ScreenIntCoord aHeight);
   void DynamicToolbarOffsetChanged(ScreenIntCoord aOffset);
   void KeyboardHeightChanged(ScreenIntCoord aHeight);
+  void AndroidPipModeChanged(bool);
 #endif
 
   void Activate(uint64_t aActionId);
diff --git a/dom/ipc/PBrowser.ipdl b/dom/ipc/PBrowser.ipdl
index 6ba0c9dad738..5a376753db92 100644
--- a/dom/ipc/PBrowser.ipdl
+++ b/dom/ipc/PBrowser.ipdl
@@ -703,6 +703,8 @@ child:
 
     async KeyboardHeightChanged(ScreenIntCoord height);
 
+    async AndroidPipModeChanged(bool aPipMode);
+
     /**
      * StopIMEStateManagement() is called when the process loses focus and
      * should stop managing IME state.
diff --git a/dom/webidl/Document.webidl b/dom/webidl/Document.webidl
index 30a30345bb29..c43f3c9e8445 100644
--- a/dom/webidl/Document.webidl
+++ b/dom/webidl/Document.webidl
@@ -385,6 +385,12 @@ partial interface Document {
   [ChromeOnly]
   readonly attribute boolean loadedFromPrototype;
 
+  // Whether we're in android's Picture-in-Picture mode.
+  // Top level document only (for now, if we want to deal with iframes, please
+  // also fix bug 1959448 while at it).
+  [ChromeOnly]
+  readonly attribute boolean inAndroidPipMode;
+
   // The principal to use for the storage area of this document
   [ChromeOnly]
   readonly attribute Principal effectiveStoragePrincipal;
diff --git a/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java b/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java
index c351139207a7..499e12bd3958 100644
--- a/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java
+++ b/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java
@@ -410,6 +410,9 @@ public class GeckoSession {
     @WrapForJNI(calledFrom = "ui", dispatchTo = "gecko")
     public native void onSafeAreaInsetsChanged(int top, int right, int bottom, int left);
 
+    @WrapForJNI(calledFrom = "ui", dispatchTo = "gecko")
+    public native void onPipModeChanged(boolean enabled);
+
     @WrapForJNI(calledFrom = "ui", dispatchTo = "gecko")
     public native void onKeyboardHeightChanged(int height);
 
diff --git a/view/nsView.cpp b/view/nsView.cpp
index 4d6c90a95f07..291f12c0f8f6 100644
--- a/view/nsView.cpp
+++ b/view/nsView.cpp
@@ -836,7 +836,7 @@ bool nsView::WindowResized(nsIWidget* aWidget, int32_t aWidth,
   return false;
 }
 
-#if defined(MOZ_WIDGET_ANDROID)
+#ifdef MOZ_WIDGET_ANDROID
 void nsView::DynamicToolbarMaxHeightChanged(ScreenIntCoord aHeight) {
   MOZ_ASSERT(XRE_IsParentProcess(),
              "Should be only called for the browser parent process");
@@ -883,6 +883,18 @@ void nsView::KeyboardHeightChanged(ScreenIntCoord aHeight) {
         return CallState::Stop;
       });
 }
+
+void nsView::AndroidPipModeChanged(bool aPipMode) {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "Should be only called for the browser parent process");
+  MOZ_ASSERT(this == mViewManager->GetRootView(),
+             "Should be called for the root view");
+  CallOnAllRemoteChildren(
+      [aPipMode](dom::BrowserParent* aBrowserParent) -> CallState {
+        aBrowserParent->AndroidPipModeChanged(aPipMode);
+        return CallState::Continue;
+      });
+}
 #endif
 
 bool nsView::RequestWindowClose(nsIWidget* aWidget) {
diff --git a/view/nsView.h b/view/nsView.h
index ddabdfafef00..b04816adb110 100644
--- a/view/nsView.h
+++ b/view/nsView.h
@@ -390,10 +390,11 @@ class nsView final : public nsIWidgetListener {
                    ByMoveToRect) override;
   bool WindowResized(nsIWidget* aWidget, int32_t aWidth,
                      int32_t aHeight) override;
-#if defined(MOZ_WIDGET_ANDROID)
+#ifdef MOZ_WIDGET_ANDROID
   void DynamicToolbarMaxHeightChanged(mozilla::ScreenIntCoord aHeight) override;
   void DynamicToolbarOffsetChanged(mozilla::ScreenIntCoord aOffset) override;
   void KeyboardHeightChanged(mozilla::ScreenIntCoord aHeight) override;
+  void AndroidPipModeChanged(bool) override;
 #endif
   bool RequestWindowClose(nsIWidget* aWidget) override;
   MOZ_CAN_RUN_SCRIPT_BOUNDARY
diff --git a/widget/android/nsWindow.cpp b/widget/android/nsWindow.cpp
index 6bcc04a9cdb1..f89d559b8252 100644
--- a/widget/android/nsWindow.cpp
+++ b/widget/android/nsWindow.cpp
@@ -1382,6 +1382,21 @@ class LayerViewSupport final
     gkWindow->UpdateDynamicToolbarMaxHeight(ScreenIntCoord(aHeight));
   }
 
+  void OnPipModeChanged(bool aPipMode) {
+    MOZ_ASSERT(NS_IsMainThread());
+    auto win(mWindow.Access());
+    if (!win) {
+      return;  // Already shut down.
+    }
+
+    nsWindow* gkWindow = win->GetNsWindow();
+    if (!gkWindow) {
+      return;
+    }
+
+    gkWindow->PipModeChanged(aPipMode);
+  }
+
   void OnKeyboardHeightChanged(int32_t aHeight) {
     MOZ_ASSERT(NS_IsMainThread());
     auto win(mWindow.Access());
@@ -3302,6 +3317,16 @@ void nsWindow::UpdateDynamicToolbarOffset(ScreenIntCoord aOffset) {
   }
 }
 
+void nsWindow::PipModeChanged(bool aPipMode) {
+  if (mWidgetListener) {
+    mWidgetListener->AndroidPipModeChanged(aPipMode);
+  }
+
+  if (mAttachedWidgetListener) {
+    mAttachedWidgetListener->AndroidPipModeChanged(aPipMode);
+  }
+}
+
 void nsWindow::KeyboardHeightChanged(ScreenIntCoord aHeight) {
   if (mWidgetListener) {
     mWidgetListener->KeyboardHeightChanged(aHeight);
diff --git a/widget/android/nsWindow.h b/widget/android/nsWindow.h
index d97064b64188..2cb3e4d7e3b1 100644
--- a/widget/android/nsWindow.h
+++ b/widget/android/nsWindow.h
@@ -257,6 +257,8 @@ class nsWindow final : public nsBaseWidget {
 
   void KeyboardHeightChanged(mozilla::ScreenIntCoord aHeight);
 
+  void PipModeChanged(bool aPipMode);
+
   mozilla::jni::NativeWeakPtr<mozilla::widget::NPZCSupport>
   GetNPZCSupportWeakPtr();
 
diff --git a/widget/nsIWidgetListener.h b/widget/nsIWidgetListener.h
index 30510ceae8e3..6323d9633180 100644
--- a/widget/nsIWidgetListener.h
+++ b/widget/nsIWidgetListener.h
@@ -81,12 +81,13 @@ class nsIWidgetListener {
    */
   virtual void SizeModeChanged(nsSizeMode aSizeMode) {}
 
-#if defined(MOZ_WIDGET_ANDROID)
+#ifdef MOZ_WIDGET_ANDROID
   virtual void DynamicToolbarMaxHeightChanged(mozilla::ScreenIntCoord aHeight) {
   }
   virtual void DynamicToolbarOffsetChanged(mozilla::ScreenIntCoord aOffset) {}
   /** Called when the software keyboard appears/disappears. */
   virtual void KeyboardHeightChanged(mozilla::ScreenIntCoord aHeight) {}
+  virtual void AndroidPipModeChanged(bool) {}
 #endif
 
   /** Called when the macOS titlebar is shown while in fullscreen. */