diff --git a/mod/forum/classes/subscriptions.php b/mod/forum/classes/subscriptions.php
index 8f7aa2fea65..ada9b4d9e96 100644
--- a/mod/forum/classes/subscriptions.php
+++ b/mod/forum/classes/subscriptions.php
@@ -825,4 +825,39 @@ class subscriptions {
         return true;
     }
 
+    /**
+     * Gets the default subscription value for the logged in user.
+     *
+     * @param \stdClass $forum The forum record
+     * @param \context $context The course context
+     * @param \cm_info $cm cm_info
+     * @param int|null $discussionid The discussion we are checking against
+     * @return bool Default subscription
+     * @throws coding_exception
+     */
+    public static function get_user_default_subscription($forum, $context, $cm, $discussionid = null) {
+        global $USER;
+        $manageactivities = has_capability('moodle/course:manageactivities', $context);
+        if (self::subscription_disabled($forum) && !$manageactivities) {
+            // User does not have permission to subscribe to this discussion at all.
+            $discussionsubscribe = false;
+        } else if (self::is_forcesubscribed($forum)) {
+            // User does not have permission to unsubscribe from this discussion at all.
+            $discussionsubscribe = true;
+        } else {
+            if (isset($discussion) && self::is_subscribed($USER->id, $forum, $discussionid, $cm)) {
+                // User is subscribed to the discussion - continue the subscription.
+                $discussionsubscribe = true;
+            } else if (!isset($discussionid) && self::is_subscribed($USER->id, $forum, null, $cm)) {
+                // Starting a new discussion, and the user is subscribed to the forum - subscribe to the discussion.
+                $discussionsubscribe = true;
+            } else {
+                // User is not subscribed to either forum or discussion. Follow user preference.
+                $discussionsubscribe = $USER->autosubscribe ? $USER->autosubscribe : false;
+            }
+        }
+
+        return $discussionsubscribe;
+    }
+
 }
diff --git a/mod/forum/externallib.php b/mod/forum/externallib.php
index a92d3bd5b05..6a0aaa0b19b 100644
--- a/mod/forum/externallib.php
+++ b/mod/forum/externallib.php
@@ -823,9 +823,14 @@ class mod_forum_external extends external_api {
         $context = context_module::instance($cm->id);
         self::validate_context($context);
 
+        $coursecontext = \context_course::instance($course->id);
+        $discussionsubscribe = \mod_forum\subscriptions::get_user_default_subscription($forum, $coursecontext,
+            $cm, null);
+
         // Validate options.
         $options = array(
-            'discussionsubscribe' => true,
+            'discussionsubscribe' => $discussionsubscribe,
+            'private'             => false,
             'inlineattachmentsid' => 0,
             'attachmentsid' => null
         );
@@ -896,9 +901,11 @@ class mod_forum_external extends external_api {
                 $completion->update_state($cm, COMPLETION_COMPLETE);
             }
 
-            $settings = new stdClass();
-            $settings->discussionsubscribe = $options['discussionsubscribe'];
-            forum_post_subscription($settings, $forum, $discussion);
+            if ($options['discussionsubscribe']) {
+                $settings = new stdClass();
+                $settings->discussionsubscribe = $options['discussionsubscribe'];
+                forum_post_subscription($settings, $forum, $discussion);
+            }
         } else {
             throw new moodle_exception('couldnotadd', 'forum');
         }
diff --git a/mod/forum/tests/externallib_test.php b/mod/forum/tests/externallib_test.php
index c095db73a22..17a8f02bc71 100644
--- a/mod/forum/tests/externallib_test.php
+++ b/mod/forum/tests/externallib_test.php
@@ -895,6 +895,92 @@ class mod_forum_external_testcase extends externallib_advanced_testcase {
 
     }
 
+    /**
+     * Test add_discussion_post and auto subscription to a discussion.
+     */
+    public function test_add_discussion_post_subscribe_discussion() {
+        global $USER;
+
+        $this->resetAfterTest(true);
+
+        self::setAdminUser();
+
+        $user = self::getDataGenerator()->create_user();
+        $admin = get_admin();
+        // Create course to add the module.
+        $course = self::getDataGenerator()->create_course(array('groupmode' => VISIBLEGROUPS, 'groupmodeforce' => 0));
+
+        $this->getDataGenerator()->enrol_user($user->id, $course->id);
+
+        // Forum with tracking off.
+        $record = new stdClass();
+        $record->course = $course->id;
+        $forum = self::getDataGenerator()->create_module('forum', $record);
+        $cm = get_coursemodule_from_id('forum', $forum->cmid, 0, false, MUST_EXIST);
+
+        // Add discussions to the forums.
+        $record = new stdClass();
+        $record->course = $course->id;
+        $record->userid = $admin->id;
+        $record->forum = $forum->id;
+        $discussion1 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_discussion($record);
+        $discussion2 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_discussion($record);
+
+        // Try to post as user.
+        self::setUser($user);
+        // Enable auto subscribe discussion.
+        $USER->autosubscribe = true;
+        // Add a discussion post in a forum discussion where the user is not subscribed (auto-subscribe preference enabled).
+        mod_forum_external::add_discussion_post($discussion1->firstpost, 'some subject', 'some text here...');
+
+        $posts = mod_forum_external::get_forum_discussion_posts($discussion1->id);
+        $posts = external_api::clean_returnvalue(mod_forum_external::get_forum_discussion_posts_returns(), $posts);
+        // We receive the discussion and the post.
+        $this->assertEquals(2, count($posts['posts']));
+        // The user should be subscribed to the discussion after adding a discussion post.
+        $this->assertTrue(\mod_forum\subscriptions::is_subscribed($user->id, $forum, $discussion1->id, $cm));
+
+        // Disable auto subscribe discussion.
+        $USER->autosubscribe = false;
+        $this->assertTrue(\mod_forum\subscriptions::is_subscribed($user->id, $forum, $discussion1->id, $cm));
+        // Add a discussion post in a forum discussion where the user is subscribed (auto-subscribe preference disabled).
+        mod_forum_external::add_discussion_post($discussion1->firstpost, 'some subject 1', 'some text here 1...');
+
+        $posts = mod_forum_external::get_forum_discussion_posts($discussion1->id);
+        $posts = external_api::clean_returnvalue(mod_forum_external::get_forum_discussion_posts_returns(), $posts);
+        // We receive the discussion and the post.
+        $this->assertEquals(3, count($posts['posts']));
+        // The user should still be subscribed to the discussion after adding a discussion post.
+        $this->assertTrue(\mod_forum\subscriptions::is_subscribed($user->id, $forum, $discussion1->id, $cm));
+
+        $this->assertFalse(\mod_forum\subscriptions::is_subscribed($user->id, $forum, $discussion2->id, $cm));
+        // Add a discussion post in a forum discussion where the user is not subscribed (auto-subscribe preference disabled).
+        mod_forum_external::add_discussion_post($discussion2->firstpost, 'some subject 2', 'some text here 2...');
+
+        $posts = mod_forum_external::get_forum_discussion_posts($discussion2->id);
+        $posts = external_api::clean_returnvalue(mod_forum_external::get_forum_discussion_posts_returns(), $posts);
+        // We receive the discussion and the post.
+        $this->assertEquals(2, count($posts['posts']));
+        // The user should still not be subscribed to the discussion after adding a discussion post.
+        $this->assertFalse(\mod_forum\subscriptions::is_subscribed($user->id, $forum, $discussion2->id, $cm));
+
+        // Passing a value for the discussionsubscribe option parameter.
+        $this->assertFalse(\mod_forum\subscriptions::is_subscribed($user->id, $forum, $discussion2->id, $cm));
+        // Add a discussion post in a forum discussion where the user is not subscribed (auto-subscribe preference disabled),
+        // and the option parameter 'discussionsubscribe' => true in the webservice.
+        $option = array('name' => 'discussionsubscribe', 'value' => true);
+        $options[] = $option;
+        mod_forum_external::add_discussion_post($discussion2->firstpost, 'some subject 2', 'some text here 2...',
+            $options);
+
+        $posts = mod_forum_external::get_forum_discussion_posts($discussion2->id);
+        $posts = external_api::clean_returnvalue(mod_forum_external::get_forum_discussion_posts_returns(), $posts);
+        // We receive the discussion and the post.
+        $this->assertEquals(3, count($posts['posts']));
+        // The user should now be subscribed to the discussion after adding a discussion post.
+        $this->assertTrue(\mod_forum\subscriptions::is_subscribed($user->id, $forum, $discussion2->id, $cm));
+    }
+
     /*
      * Test add_discussion. A basic test since all the API functions are already covered by unit tests.
      */