diff --git a/CHANGELOG.md b/CHANGELOG.md
index cbdda20..9fa0ac1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ Changelog
 - All assertion methods now return the checked value.
 - Added `notInArray` and `notOneOf`.
 - Added `isInitialized`, to check if a class property is initialized.
+- Added `notNegativeInteger` and `negativeInteger`
 
 ### Fixed
 
diff --git a/README.md b/README.md
index e81d088..bbfac0a 100644
--- a/README.md
+++ b/README.md
@@ -89,6 +89,8 @@ Method                                                   | Description
 `integer($value, $message = '')`                         | Check that a value is an integer
 `integerish($value, $message = '')`                      | Check that a value casts to an integer
 `positiveInteger($value, $message = '')`                 | Check that a value is a positive (non-zero) integer
+`negativeInteger($value, $message = '')`                 | Check that a value is a negative integer
+`notNegativeInteger($value, $message = '')`              | Check that a value is a non-negative integer
 `float($value, $message = '')`                           | Check that a value is a float
 `numeric($value, $message = '')`                         | Check that a value is numeric
 `natural($value, $message = '')`                         | Check that a value is a non-negative integer
diff --git a/src/Assert.php b/src/Assert.php
index 5a46c9b..d585aa4 100644
--- a/src/Assert.php
+++ b/src/Assert.php
@@ -117,7 +117,9 @@ public static function integerish(mixed $value, string $message = ''): int|float
      */
     public static function positiveInteger(mixed $value, string $message = ''): int
     {
-        if (!(\is_int($value) && $value > 0)) {
+        self::integer($value);
+
+        if ($value < 1) {
             static::reportInvalidArgument(\sprintf(
                 $message ?: 'Expected a positive integer. Got: %s',
                 static::valueToString($value)
@@ -127,6 +129,50 @@ public static function positiveInteger(mixed $value, string $message = ''): int
         return $value;
     }
 
+    /**
+     * @psalm-pure
+     * @psalm-assert non-negative-int $value
+     *
+     * @psalm-return non-negative-int
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function notNegativeInteger(mixed $value, string $message = ''): int
+    {
+        self::integer($value);
+
+        if ($value < 0) {
+            static::reportInvalidArgument(\sprintf(
+                $message ?: 'Expected a non negative integer. Got: %s',
+                static::valueToString($value)
+            ));
+        }
+
+        return $value;
+    }
+
+    /**
+     * @psalm-pure
+     * @psalm-assert negative-int $value
+     *
+     * @psalm-return negative-int
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function negativeInteger(mixed $value, string $message = ''): int
+    {
+        self::integer($value);
+
+        if ($value >= 0) {
+            static::reportInvalidArgument(\sprintf(
+                $message ?: 'Expected a negative integer. Got: %s',
+                static::valueToString($value)
+            ));
+        }
+
+        return $value;
+    }
+
     /**
      * @psalm-pure
      *
@@ -1741,7 +1787,7 @@ public static function propertyNotExists(mixed $classOrObject, mixed $property,
             ));
         }
 
-        return $value;
+        return $classOrObject;
     }
 
     /**
diff --git a/src/Mixin.php b/src/Mixin.php
index 8b44e6e..c076c4e 100644
--- a/src/Mixin.php
+++ b/src/Mixin.php
@@ -293,6 +293,118 @@ public static function allNullOrPositiveInteger(?iterable $value, string $messag
         return $value;
     }
 
+    /**
+     * @psalm-pure
+     *
+     * @psalm-assert non-negative-int|null $value
+     *
+     * @return non-negative-int|null
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function nullOrNotNegativeInteger(mixed $value, string $message = ''): mixed
+    {
+        null === $value || static::notNegativeInteger($value, $message);
+
+        return $value;
+    }
+
+    /**
+     * @psalm-pure
+     *
+     * @psalm-assert iterable<non-negative-int> $value
+     *
+     * @return iterable<non-negative-int>
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function allNotNegativeInteger(iterable $value, string $message = ''): iterable
+    {
+        static::isIterable($value);
+
+        foreach ($value as $entry) {
+            static::notNegativeInteger($entry, $message);
+        }
+
+        return $value;
+    }
+
+    /**
+     * @psalm-pure
+     *
+     * @psalm-assert iterable<non-negative-int|null> $value
+     *
+     * @return iterable<non-negative-int|null>
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function allNullOrNotNegativeInteger(?iterable $value, string $message = ''): ?iterable
+    {
+        static::isIterable($value);
+
+        foreach ($value as $entry) {
+            null === $entry || static::notNegativeInteger($entry, $message);
+        }
+
+        return $value;
+    }
+
+    /**
+     * @psalm-pure
+     *
+     * @psalm-assert negative-int|null $value
+     *
+     * @return negative-int|null
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function nullOrNegativeInteger(mixed $value, string $message = ''): mixed
+    {
+        null === $value || static::negativeInteger($value, $message);
+
+        return $value;
+    }
+
+    /**
+     * @psalm-pure
+     *
+     * @psalm-assert iterable<negative-int> $value
+     *
+     * @return iterable<negative-int>
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function allNegativeInteger(iterable $value, string $message = ''): iterable
+    {
+        static::isIterable($value);
+
+        foreach ($value as $entry) {
+            static::negativeInteger($entry, $message);
+        }
+
+        return $value;
+    }
+
+    /**
+     * @psalm-pure
+     *
+     * @psalm-assert iterable<negative-int|null> $value
+     *
+     * @return iterable<negative-int|null>
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function allNullOrNegativeInteger(?iterable $value, string $message = ''): ?iterable
+    {
+        static::isIterable($value);
+
+        foreach ($value as $entry) {
+            null === $entry || static::negativeInteger($entry, $message);
+        }
+
+        return $value;
+    }
+
     /**
      * @psalm-pure
      *
diff --git a/tests/AssertTest.php b/tests/AssertTest.php
index 4b0e9eb..4e801eb 100644
--- a/tests/AssertTest.php
+++ b/tests/AssertTest.php
@@ -79,6 +79,26 @@ public static function getTests(): array
             ['positiveInteger', ['0'], false],
             ['positiveInteger', [1.0], false],
             ['positiveInteger', [1.23], false],
+            ['notNegativeInteger', [123], true],
+            ['notNegativeInteger', [1], true],
+            ['notNegativeInteger', [-123], false],
+            ['notNegativeInteger', [0], true],
+            ['notNegativeInteger', [0.0], false],
+            ['notNegativeInteger', ['123'], false],
+            ['notNegativeInteger', ['-123'], false],
+            ['notNegativeInteger', ['0'], false],
+            ['notNegativeInteger', [1.0], false],
+            ['notNegativeInteger', [1.23], false],
+            ['negativeInteger', [123], false],
+            ['negativeInteger', [1], false],
+            ['negativeInteger', [-123], true],
+            ['negativeInteger', [0], false],
+            ['negativeInteger', [0.0], false],
+            ['negativeInteger', ['123'], false],
+            ['negativeInteger', ['-123'], false],
+            ['negativeInteger', ['0'], false],
+            ['negativeInteger', [1.0], false],
+            ['negativeInteger', [1.23], false],
             ['float', [1.0], true],
             ['float', [1.23], true],
             ['float', [123], false],
diff --git a/tests/static-analysis/assert-negativeInteger.php b/tests/static-analysis/assert-negativeInteger.php
new file mode 100644
index 0000000..dc1ab5c
--- /dev/null
+++ b/tests/static-analysis/assert-negativeInteger.php
@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Webmozart\Assert\Tests\StaticAnalysis;
+
+use Webmozart\Assert\Assert;
+
+/**
+ * @psalm-pure
+ *
+ * @param mixed $value
+ *
+ * @psalm-return negative-int
+ */
+function negativeInteger(mixed $value): int
+{
+    Assert::negativeInteger($value);
+
+    return $value;
+}
+
+/**
+ * @psalm-pure
+ *
+ * @param mixed $value
+ *
+ * @psalm-return negative-int|null
+ */
+function nullOrNegativeInteger(mixed $value): ?int
+{
+    Assert::nullOrNegativeInteger($value);
+
+    return $value;
+}
+
+/**
+ * @psalm-pure
+ *
+ * @param mixed $value
+ *
+ * @return iterable<negative-int>
+ */
+function allNegativeInteger(mixed $value): iterable
+{
+    Assert::allNegativeInteger($value);
+
+    return $value;
+}
+
+/**
+ * @psalm-pure
+ *
+ * @param mixed $value
+ *
+ * @return iterable<negative-int|null>
+ */
+function allNullOrNegativeInteger(mixed $value): iterable
+{
+    Assert::allNullOrNegativeInteger($value);
+
+    return $value;
+}
diff --git a/tests/static-analysis/assert-notNegativeInteger.php b/tests/static-analysis/assert-notNegativeInteger.php
new file mode 100644
index 0000000..01874bd
--- /dev/null
+++ b/tests/static-analysis/assert-notNegativeInteger.php
@@ -0,0 +1,65 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Webmozart\Assert\Tests\StaticAnalysis;
+
+use Webmozart\Assert\Assert;
+
+/**
+ * @psalm-pure
+ *
+ * @param mixed $value
+ *
+ * @psalm-return non-negative-int
+ */
+function nonNegativeInteger(mixed $value): int
+{
+    Assert::notNegativeInteger($value);
+
+    $value *= -1;
+
+    return $value;
+}
+
+/**
+ * @psalm-pure
+ *
+ * @param mixed $value
+ *
+ * @psalm-return non-negative-int|null
+ */
+function nullOrNonNegativeInteger(mixed $value): ?int
+{
+    Assert::nullOrNotNegativeInteger($value);
+
+    return $value;
+}
+
+/**
+ * @psalm-pure
+ *
+ * @param mixed $value
+ *
+ * @return iterable<non-negative-int>
+ */
+function allNonNegativeInteger(mixed $value): iterable
+{
+    Assert::allNotNegativeInteger($value);
+
+    return $value;
+}
+
+/**
+ * @psalm-pure
+ *
+ * @param mixed $value
+ *
+ * @return iterable<non-negative-int|null>
+ */
+function allNullOrNonNegativeInteger(mixed $value): iterable
+{
+    Assert::allNullOrPositiveInteger($value);
+
+    return $value;
+}