diff --git a/packages/@ember/-internals/glimmer/tests/utils/string-test.js b/packages/@ember/-internals/glimmer/tests/utils/string-test.js
index 139c6c3846d..feca7406a85 100644
--- a/packages/@ember/-internals/glimmer/tests/utils/string-test.js
+++ b/packages/@ember/-internals/glimmer/tests/utils/string-test.js
@@ -11,6 +11,19 @@ moduleFor(
       this.assert.ok(safeString instanceof SafeString, 'should be a SafeString');
     }
 
+    ['@test [deprecated] htmlSafe via string prototype should return an instance of SafeString']() {
+      if (ENV.EXTEND_PROTOTYPES.String) {
+        let safeString;
+        expectDeprecation(() => {
+          safeString = 'you need to be more <b>bold</b>'.htmlSafe();
+        }, /String prototype extensions are deprecated./);
+
+        this.assert.ok(safeString instanceof SafeString, 'should be a SafeString');
+      } else {
+        assert.expect(0);
+      }
+    }
+
     ['@test htmlSafe should return an empty string for null']() {
       let safeString = htmlSafe(null);
 
@@ -24,6 +37,30 @@ moduleFor(
       this.assert.equal(safeString instanceof SafeString, true, 'should be a SafeString');
       this.assert.equal(safeString.toString(), '', 'should return an empty string');
     }
+
+    ['@test [deprecated] htmlSafe via string prototype should return an instance of SafeString for an empty string']() {
+      if (ENV.EXTEND_PROTOTYPES.String) {
+        let safeString;
+        expectDeprecation(() => {
+          safeString = ''.htmlSafe();
+        }, /String prototype extensions are deprecated./);
+
+        this.assert.ok(safeString instanceof SafeString, 'should be a SafeString');
+      } else {
+        assert.expect(0);
+      }
+    }
+
+    ['@test [deprecated] String.prototype.htmlSafe is not modified without EXTEND_PROTOTYPES'](assert) {
+      if (!ENV.EXTEND_PROTOTYPES.String) {
+        assert.ok(
+          'undefined' === typeof String.prototype.htmlSafe,
+          'String.prototype helper disabled'
+        );
+      } else {
+        assert.expect(0);
+      }
+    }
   }
 );
 
diff --git a/packages/ember/index.js b/packages/ember/index.js
index 7709b6b9448..e1162eccb52 100644
--- a/packages/ember/index.js
+++ b/packages/ember/index.js
@@ -493,6 +493,19 @@ Ember._captureRenderTree = captureRenderTree;
 
 if (ENV.EXTEND_PROTOTYPES.String) {
   String.prototype.htmlSafe = function () {
+    deprecate(
+      `String prototype extensions are deprecated. Please import htmlSafe from '@ember/template' instead.`,
+      true,
+      {
+        id: 'ember-string.prototype-extensions',
+        for: 'ember-source',
+        since: {
+          enabled: '3.27.1',
+        },
+        until: '4.0.0',
+        url: 'https://deprecations.emberjs.com/v3.x/#toc_ember-string-htmlsafe-ishtmlsafe',
+      }
+    );
     return htmlSafe(this);
   };
 }