diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index a4b51ae9d..c1dc67155 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -909,8 +909,14 @@ class ModelSerializer(Serializer):
         info = model_meta.get_field_info(ModelClass)
         many_to_many = {}
         for field_name, relation_info in info.relations.items():
-            if relation_info.to_many and (field_name in validated_data):
-                many_to_many[field_name] = validated_data.pop(field_name)
+            if field_name in validated_data:
+                if relation_info.to_many:
+                    many_to_many[field_name] = validated_data.pop(field_name)
+                elif not isinstance(
+                        validated_data[field_name],
+                        relation_info.related_model):
+                    validated_data[relation_info.model_field.attname] = \
+                        validated_data.pop(field_name)
 
         try:
             instance = ModelClass.objects.create(**validated_data)
@@ -1368,7 +1374,7 @@ class ModelSerializer(Serializer):
             elif getattr(unique_constraint_field, 'auto_now', None):
                 default = timezone.now
             elif unique_constraint_field.has_default():
-                default = unique_constraint_field.default
+                default = unique_constraint_field.get_default()
             else:
                 default = empty
 
diff --git a/tests/models.py b/tests/models.py
index 6c9dde8fa..7ea44c650 100644
--- a/tests/models.py
+++ b/tests/models.py
@@ -61,6 +61,19 @@ class ForeignKeySource(RESTFrameworkModel):
                                on_delete=models.CASCADE)
 
 
+class ForeignKeyInUniquenessConstraintSource(RESTFrameworkModel):
+    name = models.CharField(max_length=100)
+    target = models.ForeignKey(ForeignKeyTarget,
+                               related_name='sources_with_uniqueness',
+                               help_text='Target',
+                               verbose_name='Target',
+                               default=1,
+                               on_delete=models.CASCADE)
+
+    class Meta:
+        unique_together = ('name', 'target')
+
+
 # Nullable ForeignKey
 class NullableForeignKeySource(RESTFrameworkModel):
     name = models.CharField(max_length=100)
diff --git a/tests/test_relations_pk.py b/tests/test_relations_pk.py
index 2eebe1b5c..cdc7f2aa2 100644
--- a/tests/test_relations_pk.py
+++ b/tests/test_relations_pk.py
@@ -5,8 +5,9 @@ from django.utils import six
 
 from rest_framework import serializers
 from tests.models import (
-    ForeignKeySource, ForeignKeyTarget, ManyToManySource, ManyToManyTarget,
-    NullableForeignKeySource, NullableOneToOneSource, NullableUUIDForeignKeySource,
+    ForeignKeyInUniquenessConstraintSource, ForeignKeySource, ForeignKeyTarget,
+    ManyToManySource, ManyToManyTarget, NullableForeignKeySource,
+    NullableOneToOneSource, NullableUUIDForeignKeySource,
     OneToOnePKSource, OneToOneTarget, UUIDForeignKeyTarget
 )
 
@@ -37,6 +38,12 @@ class ForeignKeySourceSerializer(serializers.ModelSerializer):
         fields = ('id', 'name', 'target')
 
 
+class ForeignKeyUniquenessSourceSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = ForeignKeyInUniquenessConstraintSource
+        fields = ('id', 'name', 'target')
+
+
 # Nullable ForeignKey
 class NullableForeignKeySourceSerializer(serializers.ModelSerializer):
     class Meta:
@@ -360,6 +367,35 @@ class PKForeignKeyTests(TestCase):
         assert 'target' not in serializer.validated_data
 
 
+class PKForeignKeyInUniquenessConstraintTests(TestCase):
+    def test_use_default_value_for_fk(self):
+        default_val = ForeignKeyInUniquenessConstraintSource._meta.get_field(
+            'target'
+        ).default
+        target = ForeignKeyTarget.objects.create(pk=default_val)
+        serializer = ForeignKeyUniquenessSourceSerializer(
+            data={'name': 'source-1'}
+        )
+        assert serializer.is_valid()
+        serializer.save()
+        source = ForeignKeyInUniquenessConstraintSource.objects.get(
+            name='source-1',
+        )
+        assert source.target == target
+
+    def test_provide_value_for_fk(self):
+        target = ForeignKeyTarget.objects.create(pk=10)
+        serializer = ForeignKeyUniquenessSourceSerializer(
+            data={'name': 'source-2', 'target': target.pk}
+        )
+        assert serializer.is_valid()
+        serializer.save()
+        source = ForeignKeyInUniquenessConstraintSource.objects.get(
+            name='source-2',
+        )
+        assert source.target == target
+
+
 class PKNullableForeignKeyTests(TestCase):
     def setUp(self):
         target = ForeignKeyTarget(name='target-1')