diff --git a/src/Plugin/StringPluralForm/FiveFormsBreton.php b/src/Plugin/StringPluralForm/FiveFormsBreton.php
new file mode 100644
index 0000000000000000000000000000000000000000..c4ade330ca3fbdb6cf19bb3c4d75d7ea106e104c
--- /dev/null
+++ b/src/Plugin/StringPluralForm/FiveFormsBreton.php
@@ -0,0 +1,62 @@
+<?php
+
+namespace Drupal\string_plural_form\Plugin\StringPluralForm;
+
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\string_plural_form\Attribute\StringPluralForm;
+use Drupal\string_plural_form\StringPluralFormBase;
+
+/**
+ * Provides a Plural Form for "Breton" language.
+ */
+#[StringPluralForm(
+  id: "five_forms_breton",
+  admin_label: new TranslatableMarkup("Five Forms Breton"),
+  description: new TranslatableMarkup("Case in the Breton language."),
+)]
+class FiveFormsBreton extends StringPluralFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getGettextNplurals(): int {
+    return 5;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getGettextPlural(): string {
+    return "(n % 10 == 1 && n % 100 != 11 && n % 100 != 71 && n % 100 != 91) ? 0 : ((n % 10 == 2 && n % 100 != 12 && n % 100 != 72 && n % 100 != 92) ? 1 : ((((n % 10 == 3 || n % 10 == 4) || n % 10 == 9) && (n % 100 < 10 || n % 100 > 19) && (n % 100 < 70 || n % 100 > 79) && (n % 100 < 90 || n % 100 > 99)) ? 2 : ((n != 0 && n % 1000000 == 0) ? 3 : 4)))";
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAssociatedLanguageCodes(): array {
+    return [
+      "br",
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIndex(int $n): int {
+    return ($n % 10 == 1 && $n % 100 != 11 && $n % 100 != 71 && $n % 100 != 91) ? 0 : (($n % 10 == 2 && $n % 100 != 12 && $n % 100 != 72 && $n % 100 != 92) ? 1 : (((($n % 10 == 3 || $n % 10 == 4) || $n % 10 == 9) && ($n % 100 < 10 || $n % 100 > 19) && ($n % 100 < 70 || $n % 100 > 79) && ($n % 100 < 90 || $n % 100 > 99)) ? 2 : (($n != 0 && $n % 1000000 == 0) ? 3 : 4)));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIndexLabel(int $index): TranslatableMarkup {
+    return match($index) {
+      0 => new TranslatableMarkup("1, 21, 31, 41..."),
+      1 => new TranslatableMarkup("2, 22, 32, 42, ..."),
+      2 => new TranslatableMarkup("3, 4, 9, 23, 24, 29, ..."),
+      3 => new TranslatableMarkup("1000000, 2000000, ..."),
+      4 => new TranslatableMarkup("0, 5~8, 10~20, 100, 1000, 10000, ..."),
+    };
+  }
+
+}
diff --git a/tests/Unit/StringPluralForm/FiveFormsBretonTest.php b/tests/Unit/StringPluralForm/FiveFormsBretonTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..a3be190b9719e5b4b72d5b8a333e982f83b06b71
--- /dev/null
+++ b/tests/Unit/StringPluralForm/FiveFormsBretonTest.php
@@ -0,0 +1,96 @@
+<?php
+
+namespace Drupal\Tests\string_plural_form\Unit\StringPluralForm;
+
+use Drupal\string_plural_form\Plugin\StringPluralForm\FiveFormsBreton;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\string_plural_form\Plugin\StringPluralForm\FiveFormsBreton
+ * @group StringPluralFormPlugins
+ */
+class FiveFormsBretonTest extends UnitTestCase {
+
+  /**
+   * Tests \Drupal\string_plural_form\Plugin\StringPluralForm\FiveFormsBreton::getGettextNplurals()
+   */
+  public function testGetGettextNplurals() {
+    $plugin = new FiveFormsBreton();
+    $this->assertEquals(5, $plugin->getGettextNplurals());
+  }
+
+  /**
+   * Tests \Drupal\string_plural_form\Plugin\StringPluralForm\FiveFormsBreton::getGettextNplurals()
+   */
+  public function testGetGettextPlural() {
+    $plugin = new FiveFormsBreton();
+    $this->assertEquals("(n % 10 == 1 && n % 100 != 11 && n % 100 != 71 && n % 100 != 91) ? 0 : ((n % 10 == 2 && n % 100 != 12 && n % 100 != 72 && n % 100 != 92) ? 1 : ((((n % 10 == 3 || n % 10 == 4) || n % 10 == 9) && (n % 100 < 10 || n % 100 > 19) && (n % 100 < 70 || n % 100 > 79) && (n % 100 < 90 || n % 100 > 99)) ? 2 : ((n != 0 && n % 1000000 == 0) ? 3 : 4)))", $plugin->getGettextPlural());
+  }
+
+  /**
+   * Tests \Drupal\string_plural_form\Plugin\StringPluralForm\FiveFormsBreton::getIndex()
+   */
+  public function testGetIndex() {
+
+    $plugin = new FiveFormsBreton();
+    $expected_forms = [
+      0 => [1, 21, 31, 41, 51, 61, 81, 101, 121, 131, 141, 151, 161, 181],
+      1 => [2, 22, 32, 42, 52, 62, 82, 102, 122, 132, 142, 152, 162, 182],
+      2 => [
+        3, 4, 9,
+        23, 24, 29,
+        33, 34, 39,
+        43, 44, 49,
+        53, 54, 59,
+        63, 64, 69,
+        83, 84, 89,
+        103, 104, 109,
+        123, 124, 129,
+        133, 134, 139,
+        143, 144, 149,
+        153, 154, 159,
+        163, 164, 169,
+        183, 184, 189,
+      ],
+      3 => [
+        1000000, 2000000, 3000000,
+        10000000, 20000000, 30000000,
+      ],
+      4 => [
+        0,
+        5, 6, 7, 8,
+        10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
+        20, 25, 26, 27, 28,
+        30, 35, 36, 37, 38,
+        40, 45, 46, 47, 48,
+        50, 55, 56, 57, 58,
+        60, 65, 66, 67, 68,
+        70, 71, 72, 73, 74,
+        75, 76, 77, 78, 79,
+        80, 85, 86, 87, 88,
+        90, 91, 92, 93, 94,
+        95, 96, 97, 98, 99,
+        100, 105, 106, 107, 108,
+        110, 111, 112, 113, 114, 115, 116, 117, 118, 119,
+        120, 125, 126, 127, 128,
+        130, 135, 136, 137, 138,
+        140, 145, 146, 147, 148,
+        150, 155, 156, 157, 158,
+        160, 165, 166, 167, 168,
+        170, 171, 172, 173, 174, 175, 176, 177, 178, 179,
+        180, 185, 186, 187, 188,
+        190, 191, 192, 193, 194, 195, 196, 197, 198, 199,
+        1000,
+        10000,
+        100000,
+      ],
+    ];
+
+    foreach ($expected_forms as $index => $numbers) {
+      foreach ($numbers as $n) {
+        $this->assertEquals($index, $plugin->getIndex($n));
+      }
+    }
+  }
+
+}