diff --git a/src/Symfony/Component/Process/Process.php b/src/Symfony/Component/Process/Process.php
index d5c697cf9c2d7..c0b638aabda1e 100644
--- a/src/Symfony/Component/Process/Process.php
+++ b/src/Symfony/Component/Process/Process.php
@@ -304,10 +304,10 @@ public function start(callable $callback = null, array $env = [])
         $descriptors = $this->getDescriptors();
 
         if ($this->env) {
-            $env += $this->env;
+            $env += '\\' === \DIRECTORY_SEPARATOR ? array_diff_ukey($this->env, $env, 'strcasecmp') : $this->env;
         }
 
-        $env += $this->getDefaultEnv();
+        $env += '\\' === \DIRECTORY_SEPARATOR ? array_diff_ukey($this->getDefaultEnv(), $env, 'strcasecmp') : $this->getDefaultEnv();
 
         if (\is_array($commandline = $this->commandline)) {
             $commandline = implode(' ', array_map([$this, 'escapeArgument'], $commandline));
@@ -1659,8 +1659,8 @@ private function replacePlaceholders(string $commandline, array $env)
     private function getDefaultEnv(): array
     {
         $env = getenv();
-        $env = array_intersect_key($env, $_SERVER) ?: $env;
+        $env = ('\\' === \DIRECTORY_SEPARATOR ? array_intersect_ukey($env, $_SERVER, 'strcasecmp') : array_intersect_key($env, $_SERVER)) ?: $env;
 
-        return $_ENV + $env;
+        return $_ENV + ('\\' === \DIRECTORY_SEPARATOR ? array_diff_ukey($env, $_ENV, 'strcasecmp') : $env);
     }
 }
diff --git a/src/Symfony/Component/Process/Tests/ProcessTest.php b/src/Symfony/Component/Process/Tests/ProcessTest.php
index 70b8e05139026..806afbab0948c 100644
--- a/src/Symfony/Component/Process/Tests/ProcessTest.php
+++ b/src/Symfony/Component/Process/Tests/ProcessTest.php
@@ -1522,6 +1522,18 @@ public function testWaitStoppedDeadProcess()
         $this->assertFalse($process->isRunning());
     }
 
+    public function testEnvCaseInsensitiveOnWindows()
+    {
+        $p = $this->getProcessForCode('print_r([$_SERVER[\'PATH\'] ?? 1, $_SERVER[\'Path\'] ?? 2]);', null, ['PATH' => 'bar/baz']);
+        $p->run(null, ['Path' => 'foo/bar']);
+
+        if ('\\' === \DIRECTORY_SEPARATOR) {
+            $this->assertSame('Array ( [0] => 1 [1] => foo/bar )', preg_replace('/\s++/', ' ', trim($p->getOutput())));
+        } else {
+            $this->assertSame('Array ( [0] => bar/baz [1] => foo/bar )', preg_replace('/\s++/', ' ', trim($p->getOutput())));
+        }
+    }
+
     /**
      * @param string|array $commandline
      * @param mixed        $input