--- a/toolkit/components/search/SearchService.jsm
+++ b/toolkit/components/search/SearchService.jsm
@@ -2243,16 +2243,34 @@ SearchService.prototype = {
     return this.defaultEngine;
   },
 
   async setDefault(engine) {
     await this.init(true);
     return this.defaultEngine = engine;
   },
 
+  get defaultPrivateEngine() {
+    return this.defaultEngine;
+  },
+
+  set defaultPrivateEngine(engine) {
+    return this.defaultEngine = engine;
+  },
+
+  async getDefaultPrivate() {
+    await this.init(true);
+    return this.defaultEngine;
+  },
+
+  async setDefaultPrivate(engine) {
+    await this.init(true);
+    return this.defaultEngine = engine;
+  },
+
   async getDefaultEngineInfo() {
     let result = {};
 
     let engine;
     try {
       engine = await this.getDefault();
     } catch (e) {
       // The defaultEngine getter will throw if there's no engine at all,

--- a/toolkit/components/search/nsISearchService.idl
+++ b/toolkit/components/search/nsISearchService.idl
@@ -295,17 +295,17 @@ interface nsISearchService : nsISupports
    *
    * @param name
    *        The search engine's name. Must be unique. Must not be null.
    *
    * @param  details
    *         An object that can contain the following fields:
    *
    *         {iconURL} Optional: A URL string pointing to the icon to be used to
-   *                   represent the engine. 
+   *                   represent the engine.
    *
    *         {alias} Optional: A unique shortcut that can be used to retrieve the
    *                 search engine.
    *
    *         {description} Optional: A description of the search engine.
    *
    *         {method} Optional: The HTTP request method used when submitting a
    *                  search query. Case insensitive value of either "get" or "post".
@@ -443,16 +443,25 @@ interface nsISearchService : nsISupports
    * default engine.
    */
   attribute nsISearchEngine defaultEngine;
 
   Promise getDefault();
   Promise setDefault(in nsISearchEngine engine);
 
   /**
+   * The currently active search engine for private browsing mode.
+   * @see defaultEngine.
+   */
+  attribute nsISearchEngine defaultPrivateEngine;
+
+  Promise getDefaultPrivate();
+  Promise setDefaultPrivate(in nsISearchEngine engine);
+
+  /**
    * Gets a representation of the default engine in an anonymized JSON
    * string suitable for recording in the Telemetry environment.
    *
    * @return an object containing anonymized info about the default engine:
    *         name, loadPath, submissionURL (for default engines).
    */
   Promise getDefaultEngineInfo();
 

new file mode 100644
--- /dev/null
+++ b/toolkit/components/search/tests/xpcshell/test_defaultPrivateEngine.js
@@ -0,0 +1,49 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * Test that defaultEngine property can be set and yields the proper events and\
+ * behavior (search results)
+ */
+
+"use strict";
+
+add_task(async function setup() {
+  useHttpServer();
+  await AddonTestUtils.promiseStartupManager();
+});
+
+add_task(async function test_defaultPrivateEngine() {
+  let search = Services.search;
+  await search.init();
+
+  let originalDefault = search.defaultPrivateEngine;
+
+  let [engine1, engine2] = await addTestEngines([
+    { name: "Test search engine", xmlFileName: "engine.xml" },
+    { name: "A second test engine", xmlFileName: "engine2.xml" },
+  ]);
+
+  search.defaultPrivateEngine = engine1;
+  Assert.equal(search.defaultPrivateEngine, engine1);
+  search.defaultPrivateEngine = engine2;
+  Assert.equal(search.defaultPrivateEngine, engine2);
+  search.defaultPrivateEngine = engine1;
+  Assert.equal(search.defaultPrivateEngine, engine1);
+
+  // Test that hiding the currently-default engine affects the defaultEngine getter
+  // We fallback first to the original default...
+  engine1.hidden = true;
+  Assert.equal(search.defaultPrivateEngine, originalDefault);
+
+  // ... and then to the first visible engine in the list, so move our second
+  // engine to that position.
+  await search.moveEngine(engine2, 0);
+  originalDefault.hidden = true;
+  Assert.equal(search.defaultPrivateEngine, engine2);
+
+  // Test that setting defaultEngine to an already-hidden engine works, but
+  // doesn't change the return value of the getter
+  search.defaultPrivateEngine = engine1;
+  Assert.equal(search.defaultPrivateEngine, engine2);
+});

--- a/toolkit/components/search/tests/xpcshell/xpcshell.ini
+++ b/toolkit/components/search/tests/xpcshell/xpcshell.ini
@@ -73,16 +73,17 @@ support-files = data/search_ignorelist.j
 [test_location_error.js]
 [test_location_malformed_json.js]
 [test_location_timeout.js]
 [test_location_timeout_xhr.js]
 [test_nodb_pluschanges.js]
 [test_save_sorted_engines.js]
 [test_pref.js]
 [test_defaultEngine.js]
+[test_defaultPrivateEngine.js]
 [test_notifications.js]
 [test_parseSubmissionURL.js]
 [test_SearchStaticData.js]
 [test_addEngine_callback.js]
 [test_multipleIcons.js]
 [test_resultDomain.js]
 [test_searchSuggest.js]
 [test_searchSuggest_cookies.js]