Al-Darbi is returning to Saudi Arabia as part of plea deal.
He pleaded guilty before a military commission in 2014 to charges stemming from an attack on a French oil tanker and he has about nine years left to serve.
The U.S. agreed to send him to a Saudi rehabilitation program in exchange for what prosecutors say was “invaluable” testimony against other prisoners held at Guantanamo.
The Pentagon announced his transfer Wednesday. There are now 40 men still held at Guantanamo.