If you are about to say 'Grand Canyon' then you are correct...kinda. You have the right name...almost, but more than likely, the wrong country.
The Grand Canyon in Arizona is the longest canyon in North America measuring 277 miles or 446 kilometers.
However, the world's longest canyon belongs to China and is called the Yarlung Zangbo Grand Canyon.
The Yarlung Zangbo Grand Canyon out canyons the Grand Canyon by 50 kilometers, coming in at 496 kilometers in length.
The Yarlung Zangbo Grand Canyon is also the world's deepest canyon reaching depths of 5,382 meters compared to 5,133 meters for Arizona's Grand Canyon.
If you happen to be from Mars then you can claim that your world's longest canyon is Valles Marineris.
The Valles Marineris canyon system covers nearly 4,500 kilometers.