diff --git a/src/ContentLengthMiddleware.php b/src/ContentLengthMiddleware.php
new file mode 100644
index 0000000..c5e484a
--- /dev/null
+++ b/src/ContentLengthMiddleware.php
@@ -0,0 +1,41 @@
+<?php
+/**
+ * @see       https://github.com/zendframework/zend-expressive-helpers for the canonical source repository
+ * @copyright Copyright (c) 2017 Zend Technologies USA Inc. (http://www.zend.com)
+ * @license   https://github.com/zendframework/zend-expressive-helpers/blob/master/LICENSE.md New BSD License
+ */
+
+namespace Zend\Expressive\Helper;
+
+use Interop\Http\ServerMiddleware\DelegateInterface;
+use Interop\Http\ServerMiddleware\MiddlewareInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Zend\Diactoros\Response\JsonResponse;
+
+/**
+ * Middleware to inject a Content-Length response header.
+ *
+ * If the response returned by a delegate does not contain a Content-Length
+ * header, and the body size is non-null, this middleware will return a new
+ * response that contains a Content-Length header based on the body size.
+ */
+class ContentLengthMiddleware implements MiddlewareInterface
+{
+    /**
+     * {@inheritDoc}
+     */
+    public function process(ServerRequestInterface $request, DelegateInterface $delegate)
+    {
+        $response = $delegate->process($request);
+        if ($response->hasHeader('Content-Length')) {
+            return $response;
+        }
+
+        $body = $response->getBody();
+        if (null === $body->getSize()) {
+            return $response;
+        }
+
+        return $response->withHeader('Content-Length', (string) $body->getSize());
+    }
+}
diff --git a/test/ContentLengthMiddlewareTest.php b/test/ContentLengthMiddlewareTest.php
new file mode 100644
index 0000000..a14e22a
--- /dev/null
+++ b/test/ContentLengthMiddlewareTest.php
@@ -0,0 +1,59 @@
+<?php
+/**
+ * @see       https://github.com/zendframework/zend-expressive-helpers for the canonical source repository
+ * @copyright Copyright (c) 2017 Zend Technologies USA Inc. (http://www.zend.com)
+ * @license   https://github.com/zendframework/zend-expressive-helpers/blob/master/LICENSE.md New BSD License
+ */
+
+namespace ZendTest\Expressive\Helper;
+
+use Interop\Http\ServerMiddleware\DelegateInterface;
+use PHPUnit\Framework\TestCase;
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Psr\Http\Message\StreamInterface;
+use Zend\Expressive\Helper\ContentLengthMiddleware;
+
+class ContentLengthMiddlewareTest extends TestCase
+{
+    public function setUp()
+    {
+        $this->response = $response = $this->prophesize(ResponseInterface::class);
+        $this->request = $request = $this->prophesize(ServerRequestInterface::class)->reveal();
+        $this->stream = $this->prophesize(StreamInterface::class);
+
+        $delegate = $this->prophesize(DelegateInterface::class);
+        $delegate->process($request)->will([$response, 'reveal']);
+        $this->delegate = $delegate->reveal();
+
+        $this->middleware = new ContentLengthMiddleware();
+    }
+
+    public function testReturnsResponseVerbatimIfContentLengthHeaderPresent()
+    {
+        $this->response->hasHeader('Content-Length')->willReturn(true);
+        $response = $this->middleware->process($this->request, $this->delegate);
+        $this->assertSame($this->response->reveal(), $response);
+    }
+
+    public function testReturnsResponseVerbatimIfContentLengthHeaderNotPresentAndBodySizeIsNull()
+    {
+        $this->stream->getSize()->willReturn(null);
+        $this->response->hasHeader('Content-Length')->willReturn(false);
+        $this->response->getBody()->will([$this->stream, 'reveal']);
+
+        $response = $this->middleware->process($this->request, $this->delegate);
+        $this->assertSame($this->response->reveal(), $response);
+    }
+
+    public function testReturnsResponseWithContentLengthHeaderBasedOnBodySize()
+    {
+        $this->stream->getSize()->willReturn(42);
+        $this->response->hasHeader('Content-Length')->willReturn(false);
+        $this->response->getBody()->will([$this->stream, 'reveal']);
+        $this->response->withHeader('Content-Length', '42')->will([$this->response, 'reveal']);
+
+        $response = $this->middleware->process($this->request, $this->delegate);
+        $this->assertSame($this->response->reveal(), $response);
+    }
+}