diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index c95b0593f..5c726dfcd 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -881,7 +881,7 @@ class ModelSerializer(Serializer):
         except KeyError:
             return ModelField(model_field=model_field, **kwargs)
 
-    def get_validation_exclusions(self):
+    def get_validation_exclusions(self, instance=None):
         """
         Return a list of field names to exclude from model validation.
         """
@@ -893,7 +893,7 @@ class ModelSerializer(Serializer):
             field_name = field.source or field_name
             if field_name in exclusions \
                 and not field.read_only \
-                and field.required \
+                and (field.required or hasattr(instance, field_name)) \
                 and not isinstance(field, Serializer):
                 exclusions.remove(field_name)
         return exclusions
@@ -908,7 +908,7 @@ class ModelSerializer(Serializer):
         the full_clean validation checking.
         """
         try:
-            instance.full_clean(exclude=self.get_validation_exclusions())
+            instance.full_clean(exclude=self.get_validation_exclusions(instance))
         except ValidationError as err:
             self._errors = err.message_dict
             return None
diff --git a/rest_framework/tests/models.py b/rest_framework/tests/models.py
index bf9883123..6c8f2342b 100644
--- a/rest_framework/tests/models.py
+++ b/rest_framework/tests/models.py
@@ -103,7 +103,7 @@ class BlogPostComment(RESTFrameworkModel):
 
 class Album(RESTFrameworkModel):
     title = models.CharField(max_length=100, unique=True)
-
+    ref = models.CharField(max_length=10, unique=True, null=True, blank=True)
 
 class Photo(RESTFrameworkModel):
     description = models.TextField()
diff --git a/rest_framework/tests/test_serializer.py b/rest_framework/tests/test_serializer.py
index 198c269f0..441630113 100644
--- a/rest_framework/tests/test_serializer.py
+++ b/rest_framework/tests/test_serializer.py
@@ -161,7 +161,7 @@ class AlbumsSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = Album
-        fields = ['title']  # lists are also valid options
+        fields = ['title', 'ref']  # lists are also valid options
 
 
 class PositiveIntegerAsChoiceSerializer(serializers.ModelSerializer):
@@ -611,12 +611,15 @@ class ModelValidationTests(TestCase):
         """
         Just check if serializers.ModelSerializer handles unique checks via .full_clean()
         """
-        serializer = AlbumsSerializer(data={'title': 'a'})
+        serializer = AlbumsSerializer(data={'title': 'a', 'ref': '1'})
         serializer.is_valid()
         serializer.save()
         second_serializer = AlbumsSerializer(data={'title': 'a'})
         self.assertFalse(second_serializer.is_valid())
-        self.assertEqual(second_serializer.errors,  {'title': ['Album with this Title already exists.']})
+        self.assertEqual(second_serializer.errors,  {'title': ['Album with this Title already exists.'],})
+        third_serializer = AlbumsSerializer(data=[{'title': 'b', 'ref': '1'}, {'title': 'c'}])
+        self.assertFalse(third_serializer.is_valid())
+        self.assertEqual(third_serializer.errors,  [{'ref': ['Album with this Ref already exists.']}, {}])
 
     def test_foreign_key_is_null_with_partial(self):
         """