diff --git a/src/ShellSort.php b/src/ShellSort.php
new file mode 100644
index 0000000..97bcbf6
--- /dev/null
+++ b/src/ShellSort.php
@@ -0,0 +1,44 @@
+<?php
+
+namespace Src;
+
+class ShellSort
+{
+    /**
+     * @param array<int,float> $arr
+     * @return array<int,float>
+     */
+    public static function sort(array $arr): array
+    {
+        $length = count($arr);
+        $stepSequence = $length;
+        do {
+            $stepSequence = self::stepSequence($stepSequence);
+            for ($i = 0; $i < $stepSequence; $i++) {
+                for ($j = $i+$stepSequence; $j < $length; $j += $stepSequence) {
+                    for ($k = $j; $k >= $stepSequence; $k -= $stepSequence) {
+                        if ($arr[$k - $stepSequence] > $arr[$k]) {
+                            [$arr[$k], $arr[$k - $stepSequence]] = [$arr[$k - $stepSequence], $arr[$k]];
+                        }
+                    }
+                }
+            }
+        } while ($stepSequence > 1);
+        return $arr;
+    }
+
+    /**
+     * Returns the next step sequence based on the current lenght/step
+     *
+     * @param int $start current step sequence or array length
+     * @return int next lower step sequence
+     */
+    private static function stepSequence(int $start): int
+    {
+        $next = 1;
+        while ($start > $new = ($next + 1) * 3) {
+            $next = $new;
+        }
+        return $next;
+    }
+}
diff --git a/tests/ShellSortTest.php b/tests/ShellSortTest.php
new file mode 100644
index 0000000..3beba15
--- /dev/null
+++ b/tests/ShellSortTest.php
@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Src\ShellSort;
+
+final class ShellSortTest extends TestCase
+{
+    #[Test]
+    #[DataProvider('arrayProvider')]
+    public function it_should_return_the_sorted_array($arr): void
+    {
+        $this->assertEquals([1,2,3,4,5,6,7,8,9], ShellSort::sort($arr));
+    }
+
+    /*
+    * @return array
+    */
+    public static function arrayProvider(): array
+    {
+        return [
+        [[1,2,3,4,5,6,7,8,9]],
+        [[9,8,7,6,5,4,3,2,1]],
+        [[1,2,3,9,8,7,6,5,4]],
+        [[9,8,7,1,2,3,4,5,6]],
+        [[9,1,8,2,7,3,6,4,5]],
+        [[1,9,2,8,3,7,4,6,5]],
+        [[6,4,1,8,3,9,2,5,7]],
+    ];
+    }
+
+    #[Test]
+    public function it_should_sort_single_element_array(): void
+    {
+        $this->assertEquals([4], ShellSort::sort([4]));
+    }
+
+    #[Test]
+    public function test_empty_array(): void
+    {
+        $this->assertEquals([], ShellSort::sort([]));
+    }
+
+    #[Test]
+    public function it_should_sort_non_consecutive_numbers_correctly(): void
+    {
+        $this->assertEquals([2,5,6,8,9], ShellSort::sort([5,9,6,2,8]));
+    }
+
+    #[Test]
+    public function it_can_sort_array_with_1000_elements(): void
+    {
+        $random = range(1, 1000);
+        shuffle($random);
+        $this->assertEquals(range(1, 1000), ShellSort::sort($random));
+    }
+}