diff --git a/src/addon/mod/forum/components/post/post.ts b/src/addon/mod/forum/components/post/post.ts
index ced5507c8..8900dc980 100644
--- a/src/addon/mod/forum/components/post/post.ts
+++ b/src/addon/mod/forum/components/post/post.ts
@@ -48,13 +48,14 @@ export class AddonModForumPostComponent implements OnInit, OnDestroy {
     @Input() trackPosts: boolean; // True if post is being tracked.
     @Input() forum: any; // The forum the post belongs to. Required for attachments and offline posts.
     @Input() accessInfo: any; // Forum access information.
-    @Input() defaultSubject: string; // Default subject to set to new posts.
+    @Input() parentSubject?: string; // Subject of parent post.
     @Input() ratingInfo?: CoreRatingInfo; // Rating info item.
     @Output() onPostChange: EventEmitter<void>; // Event emitted when a reply is posted or modified.
 
     messageControl = new FormControl();
 
     uniqueId: string;
+    defaultReplySubject: string;
     advanced = false; // Display all form fields.
     tagsEnabled: boolean;
     displaySubject = true;
@@ -89,9 +90,11 @@ export class AddonModForumPostComponent implements OnInit, OnDestroy {
         this.uniqueId = this.post.id ? 'reply' + this.post.id : 'edit' + this.post.parent;
 
         const reTranslated = this.translate.instant('addon.mod_forum.re');
-        this.displaySubject = this.post.parent == 0 ||
-            (this.post.subject != this.defaultSubject && this.post.subject != 'Re: ' + this.defaultSubject &&
-                this.post.subject != reTranslated + this.defaultSubject);
+        this.displaySubject = !this.parentSubject ||
+            (this.post.subject != this.parentSubject && this.post.subject != `Re: ${this.parentSubject}` &&
+                this.post.subject != `${reTranslated} ${this.parentSubject}`);
+        this.defaultReplySubject = (this.post.subject.startsWith('Re: ') || this.post.subject.startsWith(reTranslated))
+            ? this.post.subject : `${reTranslated} ${this.post.subject}`;
 
         this.optionsMenuEnabled = !this.post.id || (this.forumProvider.isGetDiscussionPostAvailable() &&
                     (this.forumProvider.isDeletePostAvailable() || this.forumProvider.isUpdatePostAvailable()));
@@ -145,7 +148,7 @@ export class AddonModForumPostComponent implements OnInit, OnDestroy {
 
         this.replyData.replyingTo = replyingTo || 0;
         this.replyData.isEditing = !!isEditing;
-        this.replyData.subject = subject || this.defaultSubject || '';
+        this.replyData.subject = subject || this.defaultReplySubject || '';
         this.replyData.message = message || null;
         this.replyData.files = files || [];
         this.replyData.isprivatereply = !!isPrivate;
@@ -282,6 +285,13 @@ export class AddonModForumPostComponent implements OnInit, OnDestroy {
         } else {
             // The post being replied has changed but the data will be kept.
             this.replyData.replyingTo = this.post.id;
+
+            if (this.replyData.subject == this.originalData.subject) {
+                // Update subject only if it hadn't been modified
+                this.replyData.subject = this.defaultReplySubject;
+                this.originalData.subject = this.defaultReplySubject;
+            }
+
             this.messageControl.setValue(this.replyData.message);
         }
 
diff --git a/src/addon/mod/forum/pages/discussion/discussion.html b/src/addon/mod/forum/pages/discussion/discussion.html
index 9615fd88a..577fe6c57 100644
--- a/src/addon/mod/forum/pages/discussion/discussion.html
+++ b/src/addon/mod/forum/pages/discussion/discussion.html
@@ -42,13 +42,13 @@
         </ion-card>
 
         <div *ngIf="discussion" margin-bottom class="highlight">
-            <addon-mod-forum-post [post]="discussion" [courseId]="courseId" [discussionId]="discussionId" [component]="component" [componentId]="cmId" [replyData]="replyData" [originalData]="originalData" [defaultSubject]="defaultSubject" [forum]="forum" [accessInfo]="accessInfo" [trackPosts]="trackPosts" [ratingInfo]="ratingInfo" (onPostChange)="postListChanged()"></addon-mod-forum-post>
+            <addon-mod-forum-post [post]="discussion" [courseId]="courseId" [discussionId]="discussionId" [component]="component" [componentId]="cmId" [replyData]="replyData" [originalData]="originalData" [forum]="forum" [accessInfo]="accessInfo" [trackPosts]="trackPosts" [ratingInfo]="ratingInfo" (onPostChange)="postListChanged()"></addon-mod-forum-post>
         </div>
 
         <ion-card *ngIf="sort != 'nested'">
             <ng-container *ngFor="let post of posts; first as first">
                 <ion-item-divider *ngIf="!first"></ion-item-divider>
-                <addon-mod-forum-post [post]="post" [courseId]="courseId" [discussionId]="discussionId" [component]="component" [componentId]="cmId" [replyData]="replyData" [originalData]="originalData" [defaultSubject]="defaultSubject" [forum]="forum" [accessInfo]="accessInfo" [trackPosts]="trackPosts" [ratingInfo]="ratingInfo" (onPostChange)="postListChanged()"></addon-mod-forum-post>
+                <addon-mod-forum-post [post]="post" [courseId]="courseId" [discussionId]="discussionId" [component]="component" [componentId]="cmId" [replyData]="replyData" [originalData]="originalData" [parentSubject]="postSubjects[post.parent]" [forum]="forum" [accessInfo]="accessInfo" [trackPosts]="trackPosts" [ratingInfo]="ratingInfo" (onPostChange)="postListChanged()"></addon-mod-forum-post>
             </ng-container>
         </ion-card>
 
@@ -60,7 +60,7 @@
 
         <ng-template #nestedPosts let-post="post">
             <ion-card>
-                <addon-mod-forum-post [post]="post" [courseId]="courseId" [discussionId]="discussionId" [component]="component" [componentId]="cmId" [replyData]="replyData" [originalData]="originalData" [defaultSubject]="defaultSubject" [forum]="forum" [accessInfo]="accessInfo" [trackPosts]="trackPosts" [ratingInfo]="ratingInfo" (onPostChange)="postListChanged()"></addon-mod-forum-post>
+                <addon-mod-forum-post [post]="post" [courseId]="courseId" [discussionId]="discussionId" [component]="component" [componentId]="cmId" [replyData]="replyData" [originalData]="originalData" [parentSubject]="postSubjects[post.parent]" [forum]="forum" [accessInfo]="accessInfo" [trackPosts]="trackPosts" [ratingInfo]="ratingInfo" (onPostChange)="postListChanged()"></addon-mod-forum-post>
             </ion-card>
             <div padding-left *ngIf="post.children.length && post.children[0].subject">
                 <ng-container *ngFor="let child of post.children">
diff --git a/src/addon/mod/forum/pages/discussion/discussion.ts b/src/addon/mod/forum/pages/discussion/discussion.ts
index 454a0147a..6d47d6db9 100644
--- a/src/addon/mod/forum/pages/discussion/discussion.ts
+++ b/src/addon/mod/forum/pages/discussion/discussion.ts
@@ -51,7 +51,7 @@ export class AddonModForumDiscussionPage implements OnDestroy {
     discussion: any;
     posts: any[];
     discussionLoaded = false;
-    defaultSubject: string;
+    postSubjects: { [id: string]: string };
     isOnline: boolean;
     isSplitViewOn: boolean;
     postHasOffline: boolean;
@@ -375,10 +375,6 @@ export class AddonModForumDiscussionPage implements OnDestroy {
             }).catch(() => {
                 // Ignore errors.
             }).then(() => {
-                this.defaultSubject = this.translate.instant('addon.mod_forum.re') + ' ' +
-                    (this.discussion ? this.discussion.subject : '');
-                this.replyData.subject = this.defaultSubject;
-
                 const startingPost = this.forumProvider.extractStartingPost(posts);
                 if (startingPost) {
                     // Update discussion data from first post.
@@ -388,9 +384,6 @@ export class AddonModForumDiscussionPage implements OnDestroy {
                     return Promise.reject('Invalid forum discussion.');
                 }
 
-                this.defaultSubject = this.translate.instant('addon.mod_forum.re') + ' ' + this.discussion.subject;
-                this.replyData.subject = this.defaultSubject;
-
                 if (this.discussion.userfullname && this.discussion.parent == 0 && this.forum.type == 'single') {
                     // Hide author for first post and type single.
                     this.discussion.userfullname = null;
@@ -398,6 +391,11 @@ export class AddonModForumDiscussionPage implements OnDestroy {
 
                 this.posts = posts;
                 this.ratingInfo = ratingInfo;
+                this.postSubjects = this.posts.reduce((postSubjects, post) => {
+                    postSubjects[post.id] = post.subject;
+
+                    return postSubjects;
+                }, { [this.discussion.id]: this.discussion.subject });
             });
         }).then(() => {
             if (this.forumProvider.isSetPinStateAvailableForSite()) {