A military court on Wednesday at the US naval base in Guantanamo Bay, Cuba, convicted Osama bin Laden's driver of supporting terrorism but acquitted him on a more serious charge of conspiring with al Qaeda.
It is the first verdict from the Guantanamo Bay Military tribunal established to try suspects in the so-called war on terror.
Defence lawyers said they would appeal.
A jury of military officers heard two weeks of testimony and deliberated for over eight hours before convicting Salim Hamdan on Wednesday of five charges of providing material support for terrorism.
He was found not guilty on other charges of conspiring with al Qaeda to attack civilians, destroy property, and commit murder in violation of the laws of war.
The jury will next set a sentence for Hamdan, who faces a possible maximum term of life in prison.
Hamdan, a Yemeni, who is about 40, was captured in Afghanistan in November 2001.
The White House welcomed the verdict and the Pentagon said it intended to press forward with the trials of at least 20 more detainees at Guantanamo Bay.