The court ruled that Mohamud Said Omar has no further grounds for appeal, but his lawyer said he would urge the Dutch justice minister not to sign off on any extradition order.
"We're not there yet, he's not on the plane yet," Omar's lawyer, Bart Stapert told The Associated Press. "We will do whatever we can to raise all arguments against the extradition."
Omar has spent more than a year fighting his extradition for allegedly providing al-Shabab, Somalia's al-Qaida-linked insurgency, with money that was used to buy guns.
He moved to the Netherlands in November 2008. He was arrested at a Dutch asylum seekers' center a year later at Washington's request and jailed in a high-security Dutch prison during his extradition fight.
Omar is wanted as part of an ongoing U.S. investigation into the travels of up to 20 young men who went to fight in Somalia, starting in 2007.
The Supreme Court said in a brief written ruling that there was no reason to overturn a decision last year by Rotterdam District Court that Omar could be extradited.
"The district court did it well, on good grounds," Supreme Court Judge Bon de Savornin Lohman told The AP.
Stapert said he was disappointed the court had swept aside his appeal.
"There is a lot at stake and I honestly believe the Rotterdam court misapplied the law and I had hoped that the Supreme Court would correct that," he said.
The U.S. State Department considers al-Shabab a terror group with links to al-Qaida, but Stapert argues that at the time Omar is accused of links to the group, it was fighting a "legitimate struggle" against Ethiopian forces who were brought in by Somalia's weak U.N.-backed government in late 2006 to oust an Islamic group that had controlled southern Somalia and the capital for six months.
And he said that even if U.S. allegations were proven that Omar provided backing for the group, he was by no means a high level suspect.
"This is not the terrorist we try to stop in our anti-terror policies," Stapert said. "It's a person who, even under the (U.S.) government theory, has committed no violent acts whatsoever, has provided minimal, minimal support to what could be labeled at that point in time a legitimate opposition movement."
Stapert said he hoped to visit Omar in jail on Friday to discuss his next steps.