diff --git a/ext/gd/libgd/webpimg.c b/ext/gd/libgd/webpimg.c
index 4962c22e6c9..1b160232f20 100644
--- a/ext/gd/libgd/webpimg.c
+++ b/ext/gd/libgd/webpimg.c
@@ -779,6 +779,19 @@ WebPResult WebPEncode(const uint8* Y,
 										(chunk_size >> 24) & 255 };
 	  memcpy(*p_out, kRiffHeader, kRiffHeaderSize);
 
+	  if (img_size_bytes & 1) {  /* write a padding byte */
+		const int new_size = *p_out_size_bytes + 1;
+		unsigned char* p = (unsigned char*)realloc(*p_out, new_size);
+		if (p == NULL) {
+		  free(*p_out);
+		  *p_out = NULL;
+		  *p_out_size_bytes = 0;
+		  return webp_failure;
+		}
+		p[new_size - 1] = 0;
+		*p_out_size_bytes = new_size;
+	  }
+
 	  if (psnr) {
 		*psnr = WebPGetPSNR(Y, U, V, *p_out, *p_out_size_bytes);
 	  }
diff --git a/ext/gd/tests/bug66590.phpt b/ext/gd/tests/bug66590.phpt
new file mode 100644
index 00000000000..a3c5409d6b5
--- /dev/null
+++ b/ext/gd/tests/bug66590.phpt
@@ -0,0 +1,27 @@
+--TEST--
+Bug #66590 (imagewebp() doesn't pad to even length)
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip gd extension not available');
+if (!function_exists('imagewebp')) die('skip WebP support not available');
+?>
+--FILE--
+<?php
+$filename = __DIR__ . '/bug66590.webp';
+$im = imagecreatetruecolor(75, 75);
+$red = imagecolorallocate($im, 255, 0, 0);
+imagefilledrectangle($im, 0, 0, 74, 74, $red);
+imagewebp($im, $filename);
+$stream = fopen($filename, 'rb');
+fread($stream, 4); // skip "RIFF"
+$length = fread($stream, 4);
+fclose($stream);
+$length = unpack('V', $length)[1] + 8;
+var_dump($length === filesize($filename));
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__ . '/bug66590.webp');
+?>
+--EXPECT--
+bool(true)