diff --git a/lib/private/Files/ObjectStore/S3ConnectionTrait.php b/lib/private/Files/ObjectStore/S3ConnectionTrait.php
index 8dcd57fa63ce3..f3a88d421855c 100644
--- a/lib/private/Files/ObjectStore/S3ConnectionTrait.php
+++ b/lib/private/Files/ObjectStore/S3ConnectionTrait.php
@@ -10,10 +10,12 @@
 use Aws\Credentials\CredentialProvider;
 use Aws\Credentials\Credentials;
 use Aws\Exception\CredentialsException;
+use Aws\Middleware;
 use Aws\S3\Exception\S3Exception;
 use Aws\S3\S3Client;
 use GuzzleHttp\Promise\Create;
 use GuzzleHttp\Promise\RejectedPromise;
+use GuzzleHttp\Psr7\Utils;
 use OCP\EventDispatcher\IEventDispatcher;
 use OCP\Files\ObjectStore\Events\BucketCreatedEvent;
 use OCP\Files\StorageNotAvailableException;
@@ -21,6 +23,7 @@
 use OCP\ICacheFactory;
 use OCP\ICertificateManager;
 use OCP\Server;
+use Psr\Http\Message\RequestInterface;
 use Psr\Log\LoggerInterface;
 
 trait S3ConnectionTrait {
@@ -158,6 +161,8 @@ public function getConnection() {
 		}
 		$this->connection = new S3Client($options);
 
+		$this->addDeleteObjectsContentMd5Middleware();
+
 		try {
 			$logger = Server::get(LoggerInterface::class);
 			if (!$this->connection::isBucketDnsCompatible($this->bucket)) {
@@ -219,6 +224,41 @@ private function testTimeout() {
 		}
 	}
 
+	/**
+	 * Add middleware to inject Content-MD5 header for DeleteObjects operations
+	 *
+	 * AWS SDK PHP v3.339.0+ stopped generating the Content-MD5 header for DeleteObjects operations.
+	 * However, this is still required by the `bt-blue.com` S3 provider.
+	 * This middleware automatically calculates and adds the header to comply with
+	 * AWS S3 API requirements.
+	 *
+	 * @see https://github.com/aws/aws-sdk-php/issues/3068
+	 */
+	private function addDeleteObjectsContentMd5Middleware(): void {
+		if ($this->connection === null) {
+			return;
+		}
+
+		$handlerList = $this->connection->getHandlerList();
+		$handlerList->appendBuild(
+			Middleware::mapRequest(static function (RequestInterface $request): RequestInterface {
+				// Only add Content-MD5 for DeleteObjects operations
+				if ($request->getUri()->getQuery() !== 'delete') {
+					return $request;
+				}
+
+				// Calculate MD5 of request body and add Content-MD5 header
+				if (!$request->hasHeader('Content-MD5')) {
+					$body = $request->getBody();
+					$contentMd5 = base64_encode(Utils::hash($body, 'md5', true));
+					return $request->withHeader('Content-MD5', $contentMd5);
+				}
+
+				return $request;
+			})
+		);
+	}
+
 	public static function legacySignatureProvider($version, $service, $region) {
 		switch ($version) {
 			case 'v2':
diff --git a/tests/lib/Files/ObjectStore/S3ContentMd5MiddlewareTest.php b/tests/lib/Files/ObjectStore/S3ContentMd5MiddlewareTest.php
new file mode 100644
index 0000000000000..4ec9b8c72a94e
--- /dev/null
+++ b/tests/lib/Files/ObjectStore/S3ContentMd5MiddlewareTest.php
@@ -0,0 +1,175 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\Files\ObjectStore;
+
+use GuzzleHttp\Psr7\Request;
+use GuzzleHttp\Psr7\Utils;
+use Test\TestCase;
+
+/**
+ * Test suite for S3 Content-MD5 middleware
+ * Verifies AWS SDK PHP v3.339.0+ compatibility fix for DeleteObjects operations
+ * @see https://github.com/aws/aws-sdk-php/issues/3068
+ */
+#[\PHPUnit\Framework\Attributes\Group('objectstore')]
+class S3ContentMd5MiddlewareTest extends TestCase {
+
+	/**
+	 * Helper: Apply middleware logic to a request
+	 * Mirrors the logic from S3ConnectionTrait::addDeleteObjectsContentMd5Middleware()
+	 */
+	private function applyContentMd5Middleware(Request $request): Request {
+		if ($request->getUri()->getQuery() !== 'delete') {
+			return $request;
+		}
+
+		if (!$request->hasHeader('Content-MD5')) {
+			$body = $request->getBody();
+			$contentMd5 = base64_encode(Utils::hash($body, 'md5', true));
+			return $request->withHeader('Content-MD5', $contentMd5);
+		}
+
+		return $request;
+	}
+
+	/**
+	 * Test that Content-MD5 header is added to DeleteObjects requests
+	 */
+	public function testContentMd5HeaderAddedToDeleteObjects(): void {
+		$testBody = '<?xml version="1.0" encoding="UTF-8"?><Delete xmlns="http://s3.amazonaws.com/doc/2006-03-01/"><Object><Key>test-key</Key></Object></Delete>';
+		$request = new Request('POST', 'http://s3.example.com/bucket?delete', [], $testBody);
+
+		// Calculate expected MD5
+		$expectedMd5 = base64_encode(md5($testBody, true));
+
+		// Apply middleware logic
+		$resultRequest = $this->applyContentMd5Middleware($request);
+
+		// Verify header was added
+		$this->assertTrue($resultRequest->hasHeader('Content-MD5'));
+		$this->assertEquals($expectedMd5, $resultRequest->getHeaderLine('Content-MD5'));
+	}
+
+	/**
+	 * Test that Content-MD5 header is NOT added to non-DeleteObjects requests
+	 */
+	public function testContentMd5NotAddedToNonDeleteRequests(): void {
+		$testCases = [
+			'GET request' => new Request('GET', 'http://s3.example.com/bucket/key'),
+			'PUT request' => new Request('PUT', 'http://s3.example.com/bucket/key'),
+			'HEAD request' => new Request('HEAD', 'http://s3.example.com/bucket/key'),
+			'POST with different query' => new Request('POST', 'http://s3.example.com/bucket?uploads'),
+		];
+
+		foreach ($testCases as $label => $request) {
+			$resultRequest = $this->applyContentMd5Middleware($request);
+
+			// Verify header was NOT added for non-delete requests
+			$this->assertFalse($resultRequest->hasHeader('Content-MD5'), "Content-MD5 should not be added for: $label");
+		}
+	}
+
+	/**
+	 * Test that existing Content-MD5 header is preserved
+	 */
+	public function testExistingContentMd5HeaderPreserved(): void {
+		$testBody = 'test data';
+		$existingMd5 = 'existing-md5-value';
+		$request = new Request(
+			'POST',
+			'http://s3.example.com/bucket?delete',
+			['Content-MD5' => $existingMd5],
+			$testBody
+		);
+
+		// Apply middleware logic
+		$resultRequest = $this->applyContentMd5Middleware($request);
+
+		// Verify existing header was preserved
+		$this->assertTrue($resultRequest->hasHeader('Content-MD5'));
+		$this->assertEquals($existingMd5, $resultRequest->getHeaderLine('Content-MD5'));
+	}
+
+	/**
+	 * Test MD5 calculation with various body sizes
+	 */
+	public function testMd5CalculationWithVariousSizes(): void {
+		$testBodies = [
+			'small' => 'x',
+			'medium' => str_repeat('y', 1000),
+			'large' => str_repeat('z', 10000),
+			'xml_payload' => '<?xml version="1.0"?><Delete xmlns="http://s3.amazonaws.com/doc/2006-03-01/"><Object><Key>file1.txt</Key></Object><Object><Key>file2.txt</Key></Object></Delete>',
+		];
+
+		foreach ($testBodies as $label => $body) {
+			$request = new Request('POST', 'http://s3.example.com/bucket?delete', [], $body);
+			$expectedMd5 = base64_encode(md5($body, true));
+
+			$resultRequest = $this->applyContentMd5Middleware($request);
+
+			$this->assertEquals(
+				$expectedMd5,
+				$resultRequest->getHeaderLine('Content-MD5'),
+				"MD5 mismatch for $label body size"
+			);
+		}
+	}
+
+	/**
+	 * Test MD5 header format is base64-encoded
+	 */
+	public function testMd5HeaderFormatIsBase64(): void {
+		$testBody = 'test data for base64 validation';
+		$request = new Request('POST', 'http://s3.example.com/bucket?delete', [], $testBody);
+
+		$resultRequest = $this->applyContentMd5Middleware($request);
+
+		$md5Header = $resultRequest->getHeaderLine('Content-MD5');
+
+		// Verify it's a valid base64 string
+		$this->assertNotEmpty($md5Header);
+		$this->assertEquals($md5Header, base64_encode(base64_decode($md5Header, true)));
+
+		// Verify MD5 is typically 24 chars when base64-encoded (16 bytes)
+		$this->assertEquals(24, strlen($md5Header));
+	}
+
+	/**
+	 * Test edge case: Empty body in DeleteObjects request
+	 */
+	public function testMd5CalculationWithEmptyBody(): void {
+		$request = new Request('POST', 'http://s3.example.com/bucket?delete', [], '');
+
+		$resultRequest = $this->applyContentMd5Middleware($request);
+
+		// MD5 of empty string should still produce a valid header
+		$this->assertTrue($resultRequest->hasHeader('Content-MD5'));
+		$this->assertNotEmpty($resultRequest->getHeaderLine('Content-MD5'));
+	}
+
+	/**
+	 * Test that middleware is idempotent (doesn't double-hash)
+	 */
+	public function testMiddlewareIsIdempotent(): void {
+		$testBody = 'test data';
+		$request = new Request('POST', 'http://s3.example.com/bucket?delete', [], $testBody);
+
+		// Apply middleware twice
+		$resultRequest1 = $this->applyContentMd5Middleware($request);
+		$resultRequest2 = $this->applyContentMd5Middleware($resultRequest1);
+
+		// Headers should be identical
+		$this->assertEquals(
+			$resultRequest1->getHeaderLine('Content-MD5'),
+			$resultRequest2->getHeaderLine('Content-MD5'),
+			'Middleware should be idempotent'
+		);
+	}
+}