The move marks the first time under President Donald Trump that a detainee has left the military prison.

"The Department of Defense announced today the transfer of Ahmed Mohammed Ahmed Haza al-Darbi from the detention facility at Guantanamo Bay to the government of the Kingdom of Saudi Arabia," the Pentagon said in a statement.

Darbi entered a plea deal in February 2014 that saw him admit to planning, aiding and supporting an attack on the MV Limburg, which killed a Bulgarian sailor, injured a dozen and caused a large oil spill in the Gulf of Aden.

As part of his plea, Darbi provided evidence against another Saudi Guantanamo detainee -- Abd al-Rahim al-Nashiri, who faces the death penalty on charges he masterminded the MV Limburg attack and the 2000 attack against the USS Cole in Yemen that left 17 dead.

Darbi was sentenced to 13 years in prison, starting from the date of the plea.

But the deal included a provision that after four more years at Guantanamo, he could serve the remainder of his term at a luxurious rehab center in the Saudi capital Riyadh, where former extremists are given counseling and ideological detoxification.

The remaining population at Guantanamo is now 40, but Trump has vowed to send more detainees to the prison, which is located on a US naval base on the southeastern tip of Cuba.