The judge, Navy Capt. Keith Allred, said there is credible evidence that Yemeni prisoner Salim Ahmed Hamdan was bin Laden's personal driver from 1997-2001, occasionally served as a bodyguard for the al Qaeda leader, and sometimes picked up and delivered weapons.
"The accused is an alien unlawful enemy combatant," Allred wrote in a decision issued Wednesday and released by military authorities on Thursday.
Hamdan, who U.S. military records show is about 37, faces up to life in prison if convicted by the tribunal of conspiracy and supporting terrorism.
The judge's decision was based on two days of testimony at a hearing at the Navy base in Cuba earlier this month.
Hamdan's lawyers argued he should be treated as a prisoner of war, with more legal protections than those granted to the unlawful enemy combatants who are to be tried before the military tribunals.
The U.S. holds about 290 prisoners at Guantanamo and has said it plans to bring about 80 before the tribunals - the first to be held by the United States since the World War II era.