diff --git a/mod/forum/lib.php b/mod/forum/lib.php
index 79168f557cd..d482fa53955 100644
--- a/mod/forum/lib.php
+++ b/mod/forum/lib.php
@@ -4439,7 +4439,7 @@ function forum_update_post($newpost, $mform, $unused = null) {
 
     // Last post modified tracking.
     $discussion->timemodified = $post->modified;
-    $discussion->usermodified = $USER->id;
+    $discussion->usermodified = $post->userid;
 
     if (!$post->parent) {   // Post is a discussion starter - update discussion title and times too
         $discussion->name      = $post->subject;
diff --git a/mod/forum/tests/lib_test.php b/mod/forum/tests/lib_test.php
index ef135e5eac3..82c5ffbf57a 100644
--- a/mod/forum/tests/lib_test.php
+++ b/mod/forum/tests/lib_test.php
@@ -3260,4 +3260,58 @@ class mod_forum_lib_testcase extends advanced_testcase {
             ],
         ];
     }
+
+    /**
+     * Test that {@link forum_update_post()} keeps correct forum_discussions usermodified.
+     */
+    public function test_forum_update_post_keeps_discussions_usermodified() {
+        global $DB;
+
+        $this->resetAfterTest();
+
+        // Let there be light.
+        $teacher = self::getDataGenerator()->create_user();
+        $student = self::getDataGenerator()->create_user();
+        $course = self::getDataGenerator()->create_course();
+
+        $forum = self::getDataGenerator()->create_module('forum', (object)[
+            'course' => $course->id,
+        ]);
+
+        $generator = self::getDataGenerator()->get_plugin_generator('mod_forum');
+
+        // Let the teacher start a discussion.
+        $discussion = $generator->create_discussion((object)[
+            'course' => $course->id,
+            'userid' => $teacher->id,
+            'forum' => $forum->id,
+        ]);
+
+        // On this freshly created discussion, the teacher is the author of the last post.
+        $this->assertEquals($teacher->id, $DB->get_field('forum_discussions', 'usermodified', ['id' => $discussion->id]));
+
+        // Let the student reply to the teacher's post.
+        $reply = $generator->create_post((object)[
+            'course' => $course->id,
+            'userid' => $student->id,
+            'forum' => $forum->id,
+            'discussion' => $discussion->id,
+            'parent' => $discussion->firstpost,
+        ]);
+
+        // The student should now be the last post's author.
+        $this->assertEquals($student->id, $DB->get_field('forum_discussions', 'usermodified', ['id' => $discussion->id]));
+
+        // Let the teacher edit the student's reply.
+        $this->setUser($teacher->id);
+        $newpost = (object)[
+            'id' => $reply->id,
+            'itemid' => 0,
+            'subject' => 'Amended subject',
+        ];
+        forum_update_post($newpost, null);
+
+        // The student should be still the last post's author.
+        $this->assertEquals($student->id, $DB->get_field('forum_discussions', 'usermodified', ['id' => $discussion->id]));
+    }
 }