diff --git a/src/Symfony/Component/Translation/PluralizationRules.php b/src/Symfony/Component/Translation/PluralizationRules.php
index 2a46ce094f434..e69ceabc17abc 100644
--- a/src/Symfony/Component/Translation/PluralizationRules.php
+++ b/src/Symfony/Component/Translation/PluralizationRules.php
@@ -43,7 +43,7 @@ public static function get($number, $locale/*, bool $triggerDeprecation = true*/
             $locale = 'xbr';
         }
 
-        if (\strlen($locale) > 3) {
+        if ('en_US_POSIX' !== $locale && \strlen($locale) > 3) {
             $locale = substr($locale, 0, -\strlen(strrchr($locale, '_')));
         }
 
@@ -88,6 +88,7 @@ public static function get($number, $locale/*, bool $triggerDeprecation = true*/
             case 'de':
             case 'el':
             case 'en':
+            case 'en_US_POSIX':
             case 'eo':
             case 'es':
             case 'et':
diff --git a/src/Symfony/Contracts/Translation/Test/TranslatorTest.php b/src/Symfony/Contracts/Translation/Test/TranslatorTest.php
index 196ec1859f812..35519f34b9882 100644
--- a/src/Symfony/Contracts/Translation/Test/TranslatorTest.php
+++ b/src/Symfony/Contracts/Translation/Test/TranslatorTest.php
@@ -80,6 +80,17 @@ public function testTransChoiceWithDefaultLocale($expected, $id, $number)
         $this->assertEquals($expected, $translator->trans($id, ['%count%' => $number]));
     }
 
+    /**
+     * @dataProvider getTransChoiceTests
+     */
+    public function testTransChoiceWithEnUsPosix($expected, $id, $number)
+    {
+        $translator = $this->getTranslator();
+        $translator->setLocale('en_US_POSIX');
+
+        $this->assertEquals($expected, $translator->trans($id, ['%count%' => $number]));
+    }
+
     public function testGetSetLocale()
     {
         $translator = $this->getTranslator();
@@ -296,7 +307,7 @@ public function successLangcodes()
     {
         return [
             ['1', ['ay', 'bo', 'cgg', 'dz', 'id', 'ja', 'jbo', 'ka', 'kk', 'km', 'ko', 'ky']],
-            ['2', ['nl', 'fr', 'en', 'de', 'de_GE', 'hy', 'hy_AM']],
+            ['2', ['nl', 'fr', 'en', 'de', 'de_GE', 'hy', 'hy_AM', 'en_US_POSIX']],
             ['3', ['be', 'bs', 'cs', 'hr']],
             ['4', ['cy', 'mt', 'sl']],
             ['6', ['ar']],
diff --git a/src/Symfony/Contracts/Translation/TranslatorTrait.php b/src/Symfony/Contracts/Translation/TranslatorTrait.php
index 2c09c5f14d5e5..92a6a002144ea 100644
--- a/src/Symfony/Contracts/Translation/TranslatorTrait.php
+++ b/src/Symfony/Contracts/Translation/TranslatorTrait.php
@@ -140,7 +140,7 @@ private function getPluralizationRule(float $number, string $locale): int
     {
         $number = abs($number);
 
-        switch ('pt_BR' !== $locale && \strlen($locale) > 3 ? substr($locale, 0, strrpos($locale, '_')) : $locale) {
+        switch ('pt_BR' !== $locale && 'en_US_POSIX' !== $locale && \strlen($locale) > 3 ? substr($locale, 0, strrpos($locale, '_')) : $locale) {
             case 'af':
             case 'bn':
             case 'bg':
@@ -149,6 +149,7 @@ private function getPluralizationRule(float $number, string $locale): int
             case 'de':
             case 'el':
             case 'en':
+            case 'en_US_POSIX':
             case 'eo':
             case 'es':
             case 'et':