diff --git a/src/Uri.php b/src/Uri.php
index dc21e97..7ab71f7 100644
--- a/src/Uri.php
+++ b/src/Uri.php
@@ -27,6 +27,8 @@
  * Import functions
  */
 use function getservbyname;
+use function ltrim;
+use function strncmp;
 
 /**
  * Uniform Resource Identifier
@@ -351,6 +353,25 @@ public function __toString()
 
         $path = $this->getPath();
         if ($path !== '') {
+            // https://github.com/sunrise-php/uri/issues/31
+            // https://datatracker.ietf.org/doc/html/rfc3986#section-3.3
+            //
+            // If a URI contains an authority component,
+            // then the path component must either be empty
+            // or begin with a slash ("/") character.
+            if ($authority !== '' && strncmp($path, '/', 1) !== 0) {
+                $path = '/' . $path;
+            }
+
+            // https://github.com/sunrise-php/uri/issues/31
+            // https://datatracker.ietf.org/doc/html/rfc3986#section-3.3
+            //
+            // If a URI does not contain an authority component,
+            // then the path cannot begin with two slash characters ("//").
+            if ($authority === '' && strncmp($path, '//', 2) === 0) {
+                $path = '/' . ltrim($path, '/');
+            }
+
             $uri .= $path;
         }
 
diff --git a/tests/UriTest.php b/tests/UriTest.php
index 4d54ef7..f81fc9d 100644
--- a/tests/UriTest.php
+++ b/tests/UriTest.php
@@ -567,6 +567,21 @@ public function testToString()
         $this->assertSame(self::TEST_URI, (string) $uri);
     }
 
+    public function testToStringIssue31()
+    {
+        $uri = new Uri('//username@hostname');
+        $uri = $uri->withPath('pathname');
+        $this->assertSame('//username@hostname/pathname', $uri->__toString());
+
+        $uri = new Uri('scheme:');
+        $uri = $uri->withPath('//pathname');
+        $this->assertSame('scheme:/pathname', $uri->__toString());
+
+        $uri = new Uri('scheme:');
+        $uri = $uri->withPath('///pathname');
+        $this->assertSame('scheme:/pathname', $uri->__toString());
+    }
+
     // Normalizes...
 
     public function testNormalizeScheme()