The verdict on Salim Hamdan is the first to be delivered in a full war crimes tribunal at the US prison facility in Cuba.
A jury of US military officers convicted Hamdan of providing material support for terrorism but acquitted him on additional charges of conspiring with Al-Qaeda to commit war crimes.
The prosecution said he had played a vital role in the conspiracy behind the 11 September attacks, while defence lawyers said he was a low-level employee.
The judge scheduled a sentencing hearing for this afternoon for Hamdan, who faces a maximum penalty of life in prison.
The jurors deliberated a little over eight hours before reaching their verdict.
Hamdan, a Yemeni, was captured in Afghanistan in November 2001.
The White House said he had received a fair trial.
