diff --git a/src/browser-polyfill.js b/src/browser-polyfill.js
index c6cefc9..3ae8c49 100644
--- a/src/browser-polyfill.js
+++ b/src/browser-polyfill.js
@@ -7,6 +7,13 @@
 "use strict";
 
 if (typeof browser === "undefined") {
+  const SEND_RESPONSE_DEPRECATION_WARNING = `
+      Returning a Promise is the preferred way to send a reply from an
+      onMessage/onMessageExternal listener, as the sendResponse will be
+      removed from the specs (See
+      https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/runtime/onMessage)
+    `.replace(/\s+/g, " ").trim();
+
   // Wrapping the bulk of this polyfill in a one-time-use function is a minor
   // optimization for Firefox. Since Spidermonkey does not fully parse the
   // contents of a function until the first time it's called, and since it will
@@ -338,6 +345,9 @@ if (typeof browser === "undefined") {
       },
     });
 
+    // Keep track if the deprecation warning has been logged at least once.
+    let loggedSendResponseDeprecationWarning = false;
+
     const onMessageWrappers = new DefaultWeakMap(listener => {
       if (typeof listener !== "function") {
         return listener;
@@ -366,6 +376,10 @@ if (typeof browser === "undefined") {
         let wrappedSendResponse;
         let sendResponsePromise = new Promise(resolve => {
           wrappedSendResponse = function(response) {
+            if (!loggedSendResponseDeprecationWarning) {
+              console.warn(SEND_RESPONSE_DEPRECATION_WARNING, new Error().stack);
+              loggedSendResponseDeprecationWarning = true;
+            }
             didCallSendResponse = true;
             resolve(response);
           };