diff --git a/src/SpecBaseObject.php b/src/SpecBaseObject.php
index 1de429b..5bac350 100644
--- a/src/SpecBaseObject.php
+++ b/src/SpecBaseObject.php
@@ -525,4 +525,37 @@ public function getExtensions(): array
         }
         return $extensions;
     }
+
+    /**
+     * Returns extension property with `x-` prefix.
+     * @see https://github.com/OAI/OpenAPI-Specification/blob/3.0.2/versions/3.0.2.md#specificationExtensions
+     * @param string $key The property key, e.g. 'x-prop' or 'prop' (the 'x-' prefix is added automatically if needed).
+     * @return mixed|null The property value, if present.
+     * @since 1.8.0
+     */
+    public function getExtension(string $key)
+    {
+        if (strpos($key, 'x-') !== 0) {
+            $key = 'x-' . $key;
+        }
+
+        return $this->_properties[$key] ?? null;
+    }
+
+    /**
+     * Set extension property with `x-` prefix.
+     * @see https://github.com/OAI/OpenAPI-Specification/blob/3.0.2/versions/3.0.2.md#specificationExtensions
+     * @param string $key The property key, e.g. 'x-prop' or 'prop' (the 'x-' prefix is added automatically if needed).
+     * @param mixed $value The property value.
+     * @return void
+     * @since 1.8.0
+     */
+    public function setExtension(string $key, $value): void
+    {
+        if (strpos($key, 'x-') !== 0) {
+            $key = 'x-' . $key;
+        }
+
+        $this->_properties[$key] = $value;
+    }
 }
diff --git a/tests/WriterTest.php b/tests/WriterTest.php
index fc415b5..4be35fd 100644
--- a/tests/WriterTest.php
+++ b/tests/WriterTest.php
@@ -37,6 +37,33 @@ public function testWriteJson()
         );
     }
 
+    public function testWriteJsonExtensions()
+    {
+        $openapi = $this->createOpenAPI();
+        $openapi->{'x-extra-var'} = 'foo';
+        $openapi->setExtension('x-another-var', 'bar');
+        $openapi->setExtension('short-var', 'baz');
+
+        $json = \cebe\openapi\Writer::writeToJson($openapi);
+
+        $this->assertEquals(preg_replace('~\R~', "\n", <<<JSON
+{
+    "openapi": "3.0.0",
+    "info": {
+        "title": "Test API",
+        "version": "1.0.0"
+    },
+    "paths": {},
+    "x-extra-var": "foo",
+    "x-another-var": "bar",
+    "x-short-var": "baz"
+}
+JSON
+),
+            $json
+        );
+    }
+
     public function testWriteJsonMofify()
     {
         $openapi = $this->createOpenAPI();
diff --git a/tests/spec/InfoTest.php b/tests/spec/InfoTest.php
index 932e5e0..35f8204 100644
--- a/tests/spec/InfoTest.php
+++ b/tests/spec/InfoTest.php
@@ -27,6 +27,7 @@ public function testRead()
   name: Apache 2.0
   url: https://www.apache.org/licenses/LICENSE-2.0.html
 version: 1.0.1
+x-extra-var: foo
 YAML
             , Info::class);
 
@@ -46,6 +47,11 @@ public function testRead()
         $this->assertInstanceOf(License::class, $info->license);
         $this->assertEquals('Apache 2.0', $info->license->name);
         $this->assertEquals('https://www.apache.org/licenses/LICENSE-2.0.html', $info->license->url);
+
+        $this->assertEquals('foo', $info->getExtensions()['x-extra-var']);
+        $this->assertEquals('foo', $info->getExtension('x-extra-var'));
+        $this->assertEquals('foo', $info->getExtension('extra-var'));
+        $this->assertNull($info->getExtension('another-var'));
     }
 
     public function testReadInvalid()
@@ -117,4 +123,4 @@ public function testReadInvalidLicense()
         $this->assertNull($info->contact);
 
     }
-}
\ No newline at end of file
+}