The long-awaited trial of Allen Stanford, who is accused of swindling thousands of investors out of more than $7bn through an elaborate Ponzi scheme, will start next month.
A judge in Texas ruled that Mr Stanford, 61, had faked amnesia to try to avoid trial. His defence lawyers claimed he had suffered memory loss after a fellow prisoner injured him in 2009.
Assistant US Attorney Gregg Costa had told the judge: "He wants to con his way out of this case the same way he conned investors for more than 20 years. Don't let him do it."
Prison doctors found he had "no mental illness which would interfere with his ability to understand the nature and consequences of the proceedings against him or to assist properly in his defence," according to a report for the judge.
Judge David Hittner said: "I have found a preponderance of the evidence that Stanford is competent to stand trial."
He had been declared incompetent to stand trial in January due to an addiction to an anti-anxiety drug. He spent more than eight months at a prison hospital being treated for his addiction and evaluated.
Mr Stanford, who ran the Antigua-based Stanford Financial Group, was charged with fraud, conspiracy and money laundering. He pleaded not guilty.