diff --git a/django/db/migrations/operations/models.py b/django/db/migrations/operations/models.py
index 16cadc7a2fb..8019bd72dc8 100644
--- a/django/db/migrations/operations/models.py
+++ b/django/db/migrations/operations/models.py
@@ -777,7 +777,10 @@ class AddIndex(IndexOperation):
 
     def state_forwards(self, app_label, state):
         model_state = state.models[app_label, self.model_name_lower]
-        model_state.options[self.option_name].append(self.index)
+        indexes = list(model_state.options[self.option_name])
+        indexes.append(self.index.clone())
+        model_state.options[self.option_name] = indexes
+        state.reload_model(app_label, self.model_name_lower, delay=True)
 
     def database_forwards(self, app_label, schema_editor, from_state, to_state):
         model = to_state.apps.get_model(app_label, self.model_name)
@@ -821,6 +824,7 @@ class RemoveIndex(IndexOperation):
         model_state = state.models[app_label, self.model_name_lower]
         indexes = model_state.options[self.option_name]
         model_state.options[self.option_name] = [idx for idx in indexes if idx.name != self.name]
+        state.reload_model(app_label, self.model_name_lower, delay=True)
 
     def database_forwards(self, app_label, schema_editor, from_state, to_state):
         model = from_state.apps.get_model(app_label, self.model_name)
diff --git a/django/db/migrations/state.py b/django/db/migrations/state.py
index e2253d74c4a..0ba8eea89f0 100644
--- a/django/db/migrations/state.py
+++ b/django/db/migrations/state.py
@@ -584,6 +584,9 @@ class ModelState(object):
             app_label=self.app_label,
             name=self.name,
             fields=list(self.fields),
+            # Since options are shallow-copied here, operations such as
+            # AddIndex must replace their option (e.g 'indexes') rather
+            # than mutating it.
             options=dict(self.options),
             bases=self.bases,
             managers=list(self.managers),
diff --git a/docs/releases/1.11.1.txt b/docs/releases/1.11.1.txt
index 1d0b70f76b9..b396ec70ae6 100644
--- a/docs/releases/1.11.1.txt
+++ b/docs/releases/1.11.1.txt
@@ -69,3 +69,6 @@ Bugfixes
 
 * Fixed crash in ``BaseGeometryWidget.get_context()`` when overriding existing
   ``attrs`` (:ticket:`28105`).
+
+* Prevented ``AddIndex`` and ``RemoveIndex`` from mutating model state
+  (:ticket:`28043`).
diff --git a/tests/migrations/test_operations.py b/tests/migrations/test_operations.py
index e1daabe8b52..dd8aff4ca87 100644
--- a/tests/migrations/test_operations.py
+++ b/tests/migrations/test_operations.py
@@ -1491,6 +1491,29 @@ class OperationTests(OperationTestBase):
         self.unapply_operations("test_rmin", project_state, operations=operations)
         self.assertIndexExists("test_rmin_pony", ["pink", "weight"])
 
+    def test_add_index_state_forwards(self):
+        project_state = self.set_up_test_model('test_adinsf')
+        index = models.Index(fields=['pink'], name='test_adinsf_pony_pink_idx')
+        old_model = project_state.apps.get_model('test_adinsf', 'Pony')
+        new_state = project_state.clone()
+
+        operation = migrations.AddIndex('Pony', index)
+        operation.state_forwards('test_adinsf', new_state)
+        new_model = new_state.apps.get_model('test_adinsf', 'Pony')
+        self.assertIsNot(old_model, new_model)
+
+    def test_remove_index_state_forwards(self):
+        project_state = self.set_up_test_model('test_rminsf')
+        index = models.Index(fields=['pink'], name='test_rminsf_pony_pink_idx')
+        migrations.AddIndex('Pony', index).state_forwards('test_rminsf', project_state)
+        old_model = project_state.apps.get_model('test_rminsf', 'Pony')
+        new_state = project_state.clone()
+
+        operation = migrations.RemoveIndex('Pony', 'test_rminsf_pony_pink_idx')
+        operation.state_forwards('test_rminsf', new_state)
+        new_model = new_state.apps.get_model('test_rminsf', 'Pony')
+        self.assertIsNot(old_model, new_model)
+
     def test_alter_field_with_index(self):
         """
         Test AlterField operation with an index to ensure indexes created via