diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index a80fc7996..e4432833c 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -13,7 +13,7 @@ response content is handled by parsers and renderers.
 import copy
 import inspect
 import traceback
-from collections import OrderedDict, defaultdict
+from collections import ChainMap, OrderedDict, defaultdict
 from collections.abc import Mapping
 
 from django.core.exceptions import FieldDoesNotExist, ImproperlyConfigured
@@ -215,12 +215,17 @@ class BaseSerializer(Field):
             'passed when instantiating the serializer instance.'
         )
 
-        if not hasattr(self, '_validated_data'):
-            validation_data = self.to_representation(self.instance) if self.instance else {}
-            validation_data.update(self.initial_data)
-
+        if not hasattr(self, '_validated_data'):            
+            if self.instance and getattr(getattr(self, 'Meta', None), 'validate_entire_instance', api_settings.VALIDATE_ENTIRE_INSTANCE):
+                data = ChainMap(
+                    self.initial_data,
+                    self.to_representation(self.instance)
+                )
+            else:
+                data = self.initial_data
+                
             try:
-                self._validated_data = self.run_validation(validation_data)
+                self._validated_data = self.run_validation(data)
             except ValidationError as exc:
                 self._validated_data = {}
                 self._errors = exc.detail
diff --git a/rest_framework/settings.py b/rest_framework/settings.py
index 9eb4c5653..87badbad9 100644
--- a/rest_framework/settings.py
+++ b/rest_framework/settings.py
@@ -124,6 +124,9 @@ DEFAULTS = {
         'retrieve': 'read',
         'destroy': 'delete'
     },
+    
+    # Validation
+    'VALIDATE_ENTIRE_INSTANCE': False,
 }
 
 
diff --git a/tests/test_model_serializer.py b/tests/test_model_serializer.py
index 38a9f36cc..e9d4bb06e 100644
--- a/tests/test_model_serializer.py
+++ b/tests/test_model_serializer.py
@@ -1348,6 +1348,7 @@ class Issue7489Serializer(serializers.ModelSerializer):
     class Meta:
         model = Issue7489Model
         fields = '__all__'
+        validate_entire_instance = True
 
 
 class Issue7489Test(TestCase):