diff --git a/app/assets/javascripts/discourse/models/composer.js.es6 b/app/assets/javascripts/discourse/models/composer.js.es6
index a86e48de2d3..aae9a31055f 100644
--- a/app/assets/javascripts/discourse/models/composer.js.es6
+++ b/app/assets/javascripts/discourse/models/composer.js.es6
@@ -387,9 +387,14 @@ const Composer = RestModel.extend({
     return SAVE_ICONS[action];
   },
 
-  @computed("action", "whisper")
-  saveLabel(action, whisper) {
-    return whisper ? "composer.create_whisper" : SAVE_LABELS[action];
+  @computed("action", "whisper", "editConflict")
+  saveLabel(action, whisper, editConflict) {
+    if (editConflict) {
+      return "composer.overwrite_edit";
+    } else if (whisper) {
+      return "composer.create_whisper";
+    }
+    return SAVE_LABELS[action];
   },
 
   hasMetaData: function() {
@@ -727,7 +732,8 @@ const Composer = RestModel.extend({
       composerOpened: null,
       composerTotalOpened: 0,
       featuredLink: null,
-      noBump: false
+      noBump: false,
+      editConflict: false
     });
   },
 
@@ -762,6 +768,7 @@ const Composer = RestModel.extend({
 
     const props = {
       raw: this.get("reply"),
+      raw_old: this.get("editConflict") ? null : this.get("originalText"),
       edit_reason: opts.editReason,
       image_sizes: opts.imageSizes,
       cooked: this.getCookedHtml()
@@ -769,9 +776,12 @@ const Composer = RestModel.extend({
 
     this.set("composeState", SAVING);
 
-    let rollback = throwAjaxError(() => {
+    let rollback = throwAjaxError(error => {
       post.set("cooked", oldCooked);
       this.set("composeState", OPEN);
+      if (error.jqXHR && error.jqXHR.status === 409) {
+        this.set("editConflict", true);
+      }
     });
 
     return promise
diff --git a/app/controllers/posts_controller.rb b/app/controllers/posts_controller.rb
index eb30dd808cb..640679f6550 100644
--- a/app/controllers/posts_controller.rb
+++ b/app/controllers/posts_controller.rb
@@ -200,7 +200,7 @@ class PostsController < ApplicationController
     post.image_sizes = params[:image_sizes] if params[:image_sizes].present?
 
     if !guardian.send("can_edit?", post) && post.user_id == current_user.id && post.edit_time_limit_expired?
-      return render json: { errors: [I18n.t('too_late_to_edit')] }, status: 422
+      return render_json_error(I18n.t('too_late_to_edit'))
     end
 
     guardian.ensure_can_edit!(post)
@@ -210,6 +210,11 @@ class PostsController < ApplicationController
       edit_reason: params[:post][:edit_reason]
     }
 
+    raw_old = params[:post][:raw_old]
+    if raw_old.present? && raw_old != post.raw
+      return render_json_error(I18n.t('edit_conflict'), status: 409)
+    end
+
     # to stay consistent with the create api, we allow for title & category changes here
     if post.is_first_post?
       changes[:title] = params[:title] if params[:title]
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index 7eb98f0e672..918e12d8b0f 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -1372,6 +1372,7 @@ en:
         tags_missing: "You must choose at least {{count}} tags"
 
       save_edit: "Save Edit"
+      overwrite_edit: "Overwrite Edit"
       reply_original: "Reply on Original Topic"
       reply_here: "Reply Here"
       reply: "Reply"
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index adadf4da713..1b841ad84c3 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -304,6 +304,7 @@ en:
     badge: "%{display_name} badge on %{site_title}"
 
   too_late_to_edit: "That post was created too long ago. It can no longer be edited or deleted."
+  edit_conflict: "That post was edited by another user and your changes can no longer be saved."
   revert_version_same: "The current version is same as the version you are trying to revert to."
 
   excerpt_image: "image"
diff --git a/spec/requests/posts_controller_spec.rb b/spec/requests/posts_controller_spec.rb
index 57ffcea60bc..fcf74331fbe 100644
--- a/spec/requests/posts_controller_spec.rb
+++ b/spec/requests/posts_controller_spec.rb
@@ -295,6 +295,13 @@ describe PostsController do
         expect(post.raw).to eq("edited body")
       end
 
+      it 'checks for an edit conflict' do
+        update_params[:post][:raw_old] = 'old body'
+        put "/posts/#{post.id}.json", params: update_params
+
+        expect(response.status).to eq(409)
+      end
+
       it "raises an error when the post parameter is missing" do
         update_params.delete(:post)
         put "/posts/#{post.id}.json", params: update_params
diff --git a/test/javascripts/acceptance/composer-edit-conflict-test.js.es6 b/test/javascripts/acceptance/composer-edit-conflict-test.js.es6
new file mode 100644
index 00000000000..3dc48b8f1da
--- /dev/null
+++ b/test/javascripts/acceptance/composer-edit-conflict-test.js.es6
@@ -0,0 +1,25 @@
+import { acceptance } from "helpers/qunit-helpers";
+
+acceptance("Composer - Edit conflict", {
+  loggedIn: true,
+
+  pretend(server, helper) {
+    server.put("/posts/398", () => {
+      return helper.response(409, { errors: ["edit conflict"] });
+    });
+  }
+});
+
+QUnit.test("Edit a post that causes an edit conflict", async assert => {
+  await visit("/t/internationalization-localization/280");
+  await click(".topic-post:eq(0) button.show-more-actions");
+  await click(".topic-post:eq(0) button.edit");
+  await click("#reply-control button.create");
+  assert.equal(
+    find("#reply-control button.create")
+      .text()
+      .trim(),
+    I18n.t("composer.overwrite_edit"),
+    "it shows the overwrite button"
+  );
+});