diff --git a/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_sha_mismatch.js b/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_sha_mismatch.js
new file mode 100644
index 00000000000..79f8ef408e6
--- /dev/null
+++ b/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_sha_mismatch.js
@@ -0,0 +1,16 @@
+export default {
+  name: 'MRWidgetSHAMismatch',
+  template: `
+    <div class="mr-widget-body">
+      <button
+        type="button"
+        class="btn btn-success btn-small"
+        disabled="true">
+        Merge
+      </button>
+      <span class="bold">
+        The source branch HEAD has recently changed. Please reload the page and review the changes before merging.
+      </span>
+    </div>
+  `,
+};
diff --git a/app/assets/javascripts/vue_merge_request_widget/dependencies.js b/app/assets/javascripts/vue_merge_request_widget/dependencies.js
index b2eb32ead5f..bfe30ee4c08 100644
--- a/app/assets/javascripts/vue_merge_request_widget/dependencies.js
+++ b/app/assets/javascripts/vue_merge_request_widget/dependencies.js
@@ -27,6 +27,7 @@ export { default as NothingToMergeState } from './components/states/mr_widget_no
 export { default as MissingBranchState } from './components/states/mr_widget_missing_branch';
 export { default as NotAllowedState } from './components/states/mr_widget_not_allowed';
 export { default as ReadyToMergeState } from './components/states/mr_widget_ready_to_merge';
+export { default as SHAMismatchState } from './components/states/mr_widget_sha_mismatch';
 export { default as UnresolvedDiscussionsState } from './components/states/mr_widget_unresolved_discussions';
 export { default as PipelineBlockedState } from './components/states/mr_widget_pipeline_blocked';
 export { default as PipelineFailedState } from './components/states/mr_widget_pipeline_failed';
diff --git a/app/assets/javascripts/vue_merge_request_widget/mr_widget_options.js b/app/assets/javascripts/vue_merge_request_widget/mr_widget_options.js
index 7c6c2d21714..5452e19bd8e 100644
--- a/app/assets/javascripts/vue_merge_request_widget/mr_widget_options.js
+++ b/app/assets/javascripts/vue_merge_request_widget/mr_widget_options.js
@@ -16,6 +16,7 @@ import {
   MissingBranchState,
   NotAllowedState,
   ReadyToMergeState,
+  SHAMismatchState,
   UnresolvedDiscussionsState,
   PipelineBlockedState,
   PipelineFailedState,
@@ -203,6 +204,7 @@ export default {
     'mr-widget-not-allowed': NotAllowedState,
     'mr-widget-missing-branch': MissingBranchState,
     'mr-widget-ready-to-merge': ReadyToMergeState,
+    'mr-widget-sha-mismatch': SHAMismatchState,
     'mr-widget-squash-before-merge': SquashBeforeMerge,
     'mr-widget-checking': CheckingState,
     'mr-widget-unresolved-discussions': UnresolvedDiscussionsState,
diff --git a/app/assets/javascripts/vue_merge_request_widget/stores/get_state_key.js b/app/assets/javascripts/vue_merge_request_widget/stores/get_state_key.js
index fee4113f3c8..fb78ea92da1 100644
--- a/app/assets/javascripts/vue_merge_request_widget/stores/get_state_key.js
+++ b/app/assets/javascripts/vue_merge_request_widget/stores/get_state_key.js
@@ -21,6 +21,8 @@ export default function deviseState(data) {
     return 'unresolvedDiscussions';
   } else if (this.isPipelineBlocked) {
     return 'pipelineBlocked';
+  } else if (this.hasSHAChanged) {
+    return 'shaMismatch';
   } else if (this.canBeMerged) {
     return 'readyToMerge';
   }
diff --git a/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js b/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js
index faafeae5c5b..05e67706983 100644
--- a/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js
+++ b/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js
@@ -4,6 +4,7 @@ import { getStateKey } from '../dependencies';
 export default class MergeRequestStore {
 
   constructor(data) {
+    this.startingSha = data.diff_head_sha;
     this.setData(data);
   }
 
@@ -67,6 +68,7 @@ export default class MergeRequestStore {
     this.canMerge = !!data.merge_path;
     this.canCreateIssue = currentUser.can_create_issue || false;
     this.canCancelAutomaticMerge = !!data.cancel_merge_when_pipeline_succeeds_path;
+    this.hasSHAChanged = this.sha !== this.startingSha;
     this.canBeMerged = data.can_be_merged || false;
 
     // Cherry-pick and Revert actions related
diff --git a/app/assets/javascripts/vue_merge_request_widget/stores/state_maps.js b/app/assets/javascripts/vue_merge_request_widget/stores/state_maps.js
index 625d7a01c65..605dd3a1ff4 100644
--- a/app/assets/javascripts/vue_merge_request_widget/stores/state_maps.js
+++ b/app/assets/javascripts/vue_merge_request_widget/stores/state_maps.js
@@ -16,6 +16,7 @@ const stateToComponentMap = {
   mergeWhenPipelineSucceeds: 'mr-widget-merge-when-pipeline-succeeds',
   failedToMerge: 'mr-widget-failed-to-merge',
   autoMergeFailed: 'mr-widget-auto-merge-failed',
+  shaMismatch: 'mr-widget-sha-mismatch',
 };
 
 const statesToShowHelpWidget = [
diff --git a/changelogs/unreleased/32178-prevent-merge-on-sha-change.yml b/changelogs/unreleased/32178-prevent-merge-on-sha-change.yml
new file mode 100644
index 00000000000..d3208973de6
--- /dev/null
+++ b/changelogs/unreleased/32178-prevent-merge-on-sha-change.yml
@@ -0,0 +1,4 @@
+---
+title: Add state to MR widget that prevent merges when branch changes after page load
+merge_request: 11316
+author:
diff --git a/spec/javascripts/vue_mr_widget/components/states/mr_widget_sha_mismatch_spec.js b/spec/javascripts/vue_mr_widget/components/states/mr_widget_sha_mismatch_spec.js
new file mode 100644
index 00000000000..5fb1d69a8b3
--- /dev/null
+++ b/spec/javascripts/vue_mr_widget/components/states/mr_widget_sha_mismatch_spec.js
@@ -0,0 +1,16 @@
+import Vue from 'vue';
+import shaMismatchComponent from '~/vue_merge_request_widget/components/states/mr_widget_sha_mismatch';
+
+describe('MRWidgetSHAMismatch', () => {
+  describe('template', () => {
+    const Component = Vue.extend(shaMismatchComponent);
+    const vm = new Component({
+      el: document.createElement('div'),
+    });
+    it('should have correct elements', () => {
+      expect(vm.$el.classList.contains('mr-widget-body')).toBeTruthy();
+      expect(vm.$el.querySelector('button').getAttribute('disabled')).toBeTruthy();
+      expect(vm.$el.innerText).toContain('The source branch HEAD has recently changed. Please reload the page and review the changes before merging.');
+    });
+  });
+});
diff --git a/spec/javascripts/vue_mr_widget/stores/get_state_key_spec.js b/spec/javascripts/vue_mr_widget/stores/get_state_key_spec.js
index ee944f4d4e5..9a331d99865 100644
--- a/spec/javascripts/vue_mr_widget/stores/get_state_key_spec.js
+++ b/spec/javascripts/vue_mr_widget/stores/get_state_key_spec.js
@@ -25,6 +25,9 @@ describe('getStateKey', () => {
     context.canBeMerged = true;
     expect(bound()).toEqual('readyToMerge');
 
+    context.hasSHAChanged = true;
+    expect(bound()).toEqual('shaMismatch');
+
     context.isPipelineBlocked = true;
     expect(bound()).toEqual('pipelineBlocked');
 
diff --git a/spec/javascripts/vue_mr_widget/stores/mr_widget_store_spec.js b/spec/javascripts/vue_mr_widget/stores/mr_widget_store_spec.js
new file mode 100644
index 00000000000..56dd0198ae2
--- /dev/null
+++ b/spec/javascripts/vue_mr_widget/stores/mr_widget_store_spec.js
@@ -0,0 +1,22 @@
+import MergeRequestStore from '~/vue_merge_request_widget/stores/mr_widget_store';
+import mockData from '../mock_data';
+
+describe('MergeRequestStore', () => {
+  describe('setData', () => {
+    let store;
+
+    beforeEach(() => {
+      store = new MergeRequestStore(mockData);
+    });
+
+    it('should set hasSHAChanged when the diff SHA changes', () => {
+      store.setData({ ...mockData, diff_head_sha: 'a-different-string' });
+      expect(store.hasSHAChanged).toBe(true);
+    });
+
+    it('should not set hasSHAChanged when other data changes', () => {
+      store.setData({ ...mockData, work_in_progress: !mockData.work_in_progress });
+      expect(store.hasSHAChanged).toBe(false);
+    });
+  });
+});