diff --git a/Firestore/src/Query.php b/Firestore/src/Query.php
index c743705b4f2..ccbd7cd3d66 100644
--- a/Firestore/src/Query.php
+++ b/Firestore/src/Query.php
@@ -101,6 +101,7 @@ class Query
         'array-contains' => FieldFilterOperator::ARRAY_CONTAINS,
         'array-contains-any' => FieldFilterOperator::ARRAY_CONTAINS_ANY,
         'in' => FieldFilterOperator::IN,
+        'not_in' => FieldFilterOperator::NOT_IN,
     ];
 
     private $allowedDirections = [
@@ -1024,7 +1025,7 @@ private function createFieldFilter($fieldPath, $operator, $value)
         }
 
         if ($escapedPathString === self::DOCUMENT_ID) {
-            if ($operator === FieldFilterOperator::IN) {
+            if ($operator === FieldFilterOperator::IN || $operator === FieldFilterOperator::NOT_IN) {
                 $value = array_map(function ($value) use ($basePath) {
                     return $this->createDocumentReference($basePath, $value);
                 }, (array) $value);
@@ -1051,7 +1052,7 @@ private function createFieldFilter($fieldPath, $operator, $value)
                 ]
             ];
         } else {
-            $encodedValue = $operator === FieldFilterOperator::IN
+            $encodedValue = ($operator === FieldFilterOperator::IN || $operator === FieldFilterOperator::NOT_IN)
                 ? $this->valueMapper->encodeMultiValue((array)$value)
                 : $this->valueMapper->encodeValue($value);
 
diff --git a/Firestore/tests/System/QueryTest.php b/Firestore/tests/System/QueryTest.php
index 707d3bcddb6..1c761a1d970 100644
--- a/Firestore/tests/System/QueryTest.php
+++ b/Firestore/tests/System/QueryTest.php
@@ -174,6 +174,77 @@ public function testWhereInArray()
         $this->assertContains($doc2->id(), $doc_ids);
     }
 
+    public function testWhereNotInArray()
+    {
+        $name = $this->query->name();
+        $doc1 = $this->insertDoc([
+            'foos' => ['foo', 'bar'],
+        ]);
+        $doc2 = $this->insertDoc([
+            'foos' => ['foo'],
+        ]);
+
+        $docs = self::$client->collection($name)->where('foos', 'not_in', [['foo']])->documents()->rows();
+        $this->assertCount(1, $docs);
+        $this->assertEquals($doc1->name(), $docs[0]->name());
+
+        $docs = self::$client->collection($name)->where('foos', 'not_in', [['foo'], ['foo', 'bar']])
+            ->documents()->rows();
+        $this->assertEmpty($docs);
+
+        $docs = self::$client->collection($name)->where('foos', 'not_in', [['foo', 'bar']])->documents()->rows();
+        $this->assertCount(1, $docs);
+        $this->assertEquals($doc2->name(), $docs[0]->name());
+
+        $docs = self::$client->collection($name)
+            ->where(FieldPath::documentId(), 'not_in', [$doc1->id(), $doc2->id()])
+            ->documents()
+            ->rows();
+        $this->assertEmpty($docs);
+
+        $docs = self::$client->collection($name)
+            ->where(FieldPath::documentId(), 'not_in', ['non-existent-id'])
+            ->documents()
+            ->rows();
+        $this->assertCount(2, $docs);
+        $doc_ids = array_map(function ($doc) {
+            return $doc->id();
+        }, $docs);
+        $this->assertContains($doc1->id(), $doc_ids);
+        $this->assertContains($doc2->id(), $doc_ids);
+
+        $docs = self::$client->collection($name)
+        ->where(Filter::field('foos', 'not_in', [['foo']]))->documents()->rows();
+        $this->assertCount(1, $docs);
+        $this->assertEquals($doc1->name(), $docs[0]->name());
+
+        $docs = self::$client->collection($name)
+        ->where(Filter::field('foos', 'not_in', [['foo'], ['foo', 'bar']]))->documents()->rows();
+        $this->assertEmpty($docs);
+
+        $docs = self::$client->collection($name)
+        ->where(Filter::field('foos', 'not_in', [['foo', 'bar']]))->documents()->rows();
+        $this->assertCount(1, $docs);
+        $this->assertEquals($doc2->name(), $docs[0]->name());
+
+        $docs = self::$client->collection($name)
+            ->where(Filter::field(FieldPath::documentId(), 'not_in', [$doc1->id(), $doc2->id()]))
+            ->documents()
+            ->rows();
+        $this->assertEmpty($docs);
+
+        $docs = self::$client->collection($name)
+            ->where(Filter::field(FieldPath::documentId(), 'not_in', ['non-existent-id']))
+            ->documents()
+            ->rows();
+        $this->assertCount(2, $docs);
+        $doc_ids = array_map(function ($doc) {
+            return $doc->id();
+        }, $docs);
+        $this->assertContains($doc1->id(), $doc_ids);
+        $this->assertContains($doc2->id(), $doc_ids);
+    }
+
     public function testWhereWithCompositeFilter()
     {
         $name = $this->query->name();
diff --git a/Firestore/tests/Unit/QueryTest.php b/Firestore/tests/Unit/QueryTest.php
index 10e25ce74e6..5fff8ad9dba 100644
--- a/Firestore/tests/Unit/QueryTest.php
+++ b/Firestore/tests/Unit/QueryTest.php
@@ -448,6 +448,8 @@ public function whereOperators()
             ['array-contains'],
             ['in'],
             ['IN'],
+            ['not_in'],
+            ['NOT_IN'],
             ['array-contains-any'],
         ]);
     }