diff --git a/dom/push/PushManager.cpp b/dom/push/PushManager.cpp
index 4534291e429c..878519284734 100644
--- a/dom/push/PushManager.cpp
+++ b/dom/push/PushManager.cpp
@@ -419,6 +419,41 @@ bool PushManager::IsEnabled(JSContext* aCx, JSObject* aGlobal) {
   return StaticPrefs::dom_push_enabled() && ServiceWorkersEnabled(aCx, aGlobal);
 }
 
+// https://w3c.github.io/push-api/#dom-pushmanager-supportedcontentencodings
+void PushManager::GetSupportedContentEncodings(
+    GlobalObject& aGlobal, JS::MutableHandle<JSObject*> aEncodings,
+    ErrorResult& aRv) {
+  JSContext* cx = aGlobal.Context();
+
+  nsTArray<nsString> encodings{u"aes128gcm"_ns};
+  if (StaticPrefs::dom_push_indicate_aesgcm_support_enabled()) {
+    // The spec does not define orders, but Chrome is returning ["aes128gcm",
+    // "aesgcm"] and there are a bunch of code like below, which is copypasted
+    // from Minishlink/web-push-php-example endorsed by
+    // web-push-libs/web-push-php. Which means practically the preferred
+    // algorithm should go to the first place.
+    //
+    // (PushManager.supportedContentEncodings || ['aesgcm'])[0];
+    encodings.AppendElement(u"aesgcm"_ns);
+  }
+
+  JS::Rooted<JS::Value> encodingsValue(cx);
+  if (!ToJSValue(cx, encodings, &encodingsValue)) {
+    if (JS_IsThrowingOutOfMemory(cx)) {
+      MOZ_CRASH("Out of memory");
+    } else {
+      aRv.NoteJSContextException(cx);
+      return;
+    }
+  };
+  JS::Rooted<JSObject*> object(cx, &encodingsValue.toObject());
+  if (!JS_FreezeObject(cx, object)) {
+    aRv.NoteJSContextException(cx);
+    return;
+  }
+  aEncodings.set(object);
+}
+
 already_AddRefed<Promise> PushManager::Subscribe(
     const PushSubscriptionOptionsInit& aOptions, ErrorResult& aRv) {
   if (mImpl) {
diff --git a/dom/push/PushManager.h b/dom/push/PushManager.h
index f951e0a2baa7..80a6aee9172a 100644
--- a/dom/push/PushManager.h
+++ b/dom/push/PushManager.h
@@ -83,6 +83,12 @@ class PushManager final : public nsISupports, public nsWrapperCache {
       SubscriptionAction aAction, const PushSubscriptionOptionsInit& aOptions,
       ErrorResult& aRv);
 
+  // Web IDL members:
+
+  static void GetSupportedContentEncodings(
+      GlobalObject& aGlobal, JS::MutableHandle<JSObject*> aEncodings,
+      ErrorResult& aRv);
+
   already_AddRefed<Promise> Subscribe(
       const PushSubscriptionOptionsInit& aOptions, ErrorResult& aRv);
 
diff --git a/dom/webidl/PushManager.webidl b/dom/webidl/PushManager.webidl
index ff99e294b535..285688ea7070 100644
--- a/dom/webidl/PushManager.webidl
+++ b/dom/webidl/PushManager.webidl
@@ -31,6 +31,14 @@ interface PushManager {
   [Throws, ChromeOnly]
   constructor(DOMString scope);
 
+  // TODO: Use FrozenArray once available. (Bug 1236777)
+  // [SameObject] static readonly attribute FrozenArray<DOMString> supportedContentEncodings;
+  // XXX: We can't use sequence here either:
+  // 1. [Cached] is not supported for static members (Bug 1363870)
+  // 2. [Cached] is required for sequence typed attributes
+  // [Frozen, Cached, Pure] static readonly attribute sequence<DOMString> supportedContentEncodings;
+  [Throws] static readonly attribute object supportedContentEncodings;
+
   [Throws, UseCounter]
   Promise<PushSubscription>    subscribe(optional PushSubscriptionOptionsInit options = {});
   [Throws]
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index 36bb5bfd8ed0..f244254da60b 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -3490,6 +3490,15 @@
   value: true
   mirror: always
 
+# Indicate the deprecated aesgcm support in PushManager.supportedContentEncodings.
+#
+# This does not affect actual aesgcm support, any new and existing subscriptions
+# can still use aesgcm regardless of this pref.
+- name: dom.push.indicate_aesgcm_support.enabled
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
+
 # Preference that is primarily used for testing of problematic file paths.
 # It can also be used for switching between different storage directories, but
 # such feature is not officially supported.
diff --git a/testing/web-platform/meta/push-api/supported-encodings.https.any.js.ini b/testing/web-platform/meta/push-api/supported-encodings.https.any.js.ini
new file mode 100644
index 000000000000..0654b7be82e5
--- /dev/null
+++ b/testing/web-platform/meta/push-api/supported-encodings.https.any.js.ini
@@ -0,0 +1,18 @@
+[supported-encodings.https.any.html]
+  [supportedContentEncodings must be FrozenArray]
+    expected: FAIL
+
+
+[supported-encodings.https.any.sharedworker.html]
+  [supportedContentEncodings must be FrozenArray]
+    expected: FAIL
+
+
+[supported-encodings.https.any.serviceworker.html]
+  [supportedContentEncodings must be FrozenArray]
+    expected: FAIL
+
+
+[supported-encodings.https.any.worker.html]
+  [supportedContentEncodings must be FrozenArray]
+    expected: FAIL
diff --git a/testing/web-platform/tests/push-api/supported-encodings.https.any.js b/testing/web-platform/tests/push-api/supported-encodings.https.any.js
new file mode 100644
index 000000000000..45b5ed1cba46
--- /dev/null
+++ b/testing/web-platform/tests/push-api/supported-encodings.https.any.js
@@ -0,0 +1,26 @@
+// META: global=window,worker
+
+"use strict"
+// (for FrozenArray assign test)
+
+test(() => {
+  assert_true(
+    PushManager.supportedContentEncodings.includes("aes128gcm"),
+    "PushManager.supportedContentEncodings must include aes128gcm"
+  );
+}, "aes128gcm must be supported");
+
+test(() => {
+  assert_throws_js(
+    TypeError,
+    () => PushManager.supportedContentEncodings[0] = "aes1024gcm",
+    "supportedContentEncodings must be frozen"
+  );
+
+  // Intentionally not using assert_array_equals to check same-object
+  assert_equals(
+    PushManager.supportedContentEncodings,
+    PushManager.supportedContentEncodings,
+    "supportedContentEncodings must be cached"
+  );
+}, "supportedContentEncodings must be FrozenArray")