diff --git a/lib/Assert/AssertionChain.php b/lib/Assert/AssertionChain.php
index dbb6e839..d5017b64 100644
--- a/lib/Assert/AssertionChain.php
+++ b/lib/Assert/AssertionChain.php
@@ -142,6 +142,13 @@ class AssertionChain
      */
     private $all = false;
 
+    /**
+     * Perform a negative assertion.
+     *
+     * @var bool
+     */
+    private $not = false;
+
     /** @var string|Assertion Class to use for assertion calls */
     private $assertionClassName = 'Assert\Assertion';
 
@@ -197,6 +204,10 @@ public function __call($methodName, $args): AssertionChain
             }
         }
 
+        if ($this->not) {
+            $methodName = 'not'.$methodName;
+        }
+
         if ($this->all) {
             $methodName = 'all'.$methodName;
         }
@@ -218,6 +229,18 @@ public function all(): AssertionChain
         return $this;
     }
 
+    /**
+     * Switch chain into negative mode.
+     *
+     * @return \Assert\AssertionChain
+     */
+    public function not()
+    {
+        $this->not = true;
+
+        return $this;
+    }
+
     /**
      * Switch chain into mode allowing nulls, ignoring further assertions.
      *
diff --git a/tests/Assert/Tests/AssertionChainFunctionsTest.php b/tests/Assert/Tests/AssertionChainFunctionsTest.php
index e971db4d..e74f93ed 100644
--- a/tests/Assert/Tests/AssertionChainFunctionsTest.php
+++ b/tests/Assert/Tests/AssertionChainFunctionsTest.php
@@ -51,6 +51,11 @@ public function testThatAssertionChainFunctionsValidatesAllInputs()
         $this->assertInstanceOf(AssertionChain::class, \Assert\that([1, 2, 3])->all()->integer());
     }
 
+    public function testThatAssertionChainFunctionsValidateNegativeAssertions()
+    {
+        $this->assertInstanceOf(AssertionChain::class, \Assert\that('foo')->not()->isInstanceOf(\stdClass::class));
+    }
+
     public function testAssertionChainFunctionsThatAllShortcut()
     {
         $this->assertInstanceOf(AssertionChain::class, \Assert\thatAll([1, 2, 3])->integer());
diff --git a/tests/Assert/Tests/AssertionChainTest.php b/tests/Assert/Tests/AssertionChainTest.php
index bf54c61b..a23475ce 100644
--- a/tests/Assert/Tests/AssertionChainTest.php
+++ b/tests/Assert/Tests/AssertionChainTest.php
@@ -45,6 +45,11 @@ public function testThatAssertionChainSkipAssertionsOnValidNull()
         $this->assertInstanceOf(AssertionChain::class, Assert::that(null)->nullOr()->integer()->eq(10));
     }
 
+    public function testThatAssertionChainDoesANegativeAssert()
+    {
+        $this->assertInstanceOf(AssertionChain::class, Assert::that(1)->not()->eq(10));
+    }
+
     public function testThatAssertionChainValidatesAllInputs()
     {
         $this->assertInstanceOf(AssertionChain::class, Assert::that([1, 2, 3])->all()->integer());