The Supreme Court has rejected the appeal of Usama bin Laden's one-time assistant and propagandist, leaving intact his conspiracy conviction before a military tribunal.
The court issued its brief order on Monday, without giving a reason for its decision.
A military commission had convicted Ali Hamza al-Bahlul in 2008.
At issue was the government's power to use those special military courts to try war crimes, both before and after the 9/11 terror attacks.
Bahlul's lawyers had argued supposedly "domestic crimes" like conspiracy cannot be tried in the military commission system.
The defendant was charged with providing material support to terrorism and conspiracy for the Al Qaeda terror network, offenses that were not included as crimes before the Military Commissions Act went into effect several years later. That law established the tribunal system used to try terror suspects held at Guantanamo, outside the civilian federal trial courts.
Bahlul also claimed the charges against him were not recognized as "war crimes" under international law and could not be applied retroactively.
The "ex post facto" provision of the Constitution says the government may not retroactively criminalize conduct that was either legal or not on the record when committed.
A previous appeals panel had initially concluded only pre-existing federal offenses and violations of the international law of war could be used to prosecute the men.
But a federal appeals court in Washington later upheld Bahlul's conviction.
Bahlul is one of the few Guantanamo inmates convicted at the military tribunals. At least five other men had separately pleaded guilty to offenses.
Other cases, including alleged 9/11 mastermind Khalid Sheikh Mohammed and other so-called "high-value detainees," are still awaiting trial.
Yemeni native Bahlul was convicted in November 2008 of conspiring with Al Qaeda, soliciting murder and providing material support for terrorism. He was accused of making videos for the terror organization.
The high court is still considering the pending appeal of another military detainee making similar legal claims, Abd al-Rahim al-Nashiri.