One week after a judge set him free, former teen terrorist Omar Khadr has won yet another legal battle with the Conservative government, after the Supreme Court of Canada ruled that he was sentenced as a juvenile, not an adult.

While the ruling doesn't affect the 28-year-old's freedom, it means that if he is returned to jail, it would be to a provincial reformatory, rather than a federal prison.

But the symbolism looms larger than the practical effects. The Canadian government was intent on demonstrating, as it has since the United States military captured Mr. Khadr on an Afghan battlefield when he was 15, and accused him of throwing a grenade that killed a soldier, that it believes he should be punished as severely as the law allows. And the Supreme Court said that the law is much less severe than the federal government thinks it is. Adding insult, it needed only a few minutes after the hearing ended to deliver its unanimous ruling from the bench. Most rulings come several weeks after a case is heard.

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"They're saying, 'That's pretty obvious, isn't it?'" Wayne MacKay, a law professor at Dalhousie University, said of court rulings made during or immediately after a hearing.

Chief Justice Beverley McLachlin said it was a straightforward matter: A U.S. military commission sentenced Mr. Khadr to eight years in prison for the war crime of murder, and the mandatory adult penalty for murder in Canada is life. Therefore it had to be a juvenile penalty. The Canadian government argued that Mr. Khadr had received five concurrent penalties of eight years for murder and other charges. The judges expressed bafflement at the argument.

"We can't slice and dice the eight years," said Justice Marshall Rothstein, a conservative member of the court.

It was the third time the Toronto-born Mr. Khadr got his name on a Supreme Court ruling – which tied Henry Morgentaler for the most Supreme Court appeals.

In 2008, Mr. Khadr defeated the Canadian government at the Supreme Court. It ruled unanimously that the government had to disclose all records of interviews conducted by Canadian officials with him, and information given to U.S. authorities. He was being held by the U.S. government at its prison for suspected terrorists in Guantanamo Bay, Cuba.

In 2010, the court ruled partly in the government's favour and partly in Mr. Khadr's. It said unanimously that Canadian interrogations of Mr. Khadr, in the absence of counsel, after he had suffered three weeks of sleep deprivation while in U.S. custody, "offends the most basic Canadian standards about the treatment of detained youth suspects." But the court also said it could not force the government to ask the U.S. to send him home to Canada.

Last week, an Alberta Court of Appeal judge rejected a last-ditch attempt by the federal government to deny Mr. Khadr bail while he awaits an appeal of his U.S. convictions. A U.S. military commission found him guilty in 2010 of the war crime of murder and several other charges. The government argued freeing him would irreparably harm relations with the U.S., but Justice Myra Bielby said it had provided no evidence of that from U.S. authorities.

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Pollster Nik Nanos said the string of losses for the Conservative government may simply reinforce Canadians' feelings on the issue, no matter where they stand. "I don't think the Conservatives consider this a significant defeat," he said in an interview. "What the Conservatives want to convey is that they are tough on terror." He said that, while he has not done polling on attitudes toward Omar Khadr, "Canadians support a fairly hard line against anyone perceived to be engaged in promoting terror."

Mr. Khadr's lawyer, Dennis Edney, accused the government after the ruling of persecuting his client to make a political point. The Alberta Court of Appeal had said that the government's implicit position was that eight years was too soft a sentence for Mr. Khadr's crimes.

Mr. Edney told reporters that Mr. Khadr is enjoying his first few days of freedom.

"He has been embraced by the community, he has a new bike. He is happy to be out. It will be a slow journey."

With a report from Colin Freeze in Ottawa

This story corrects an earlier version that stated Mr. Khadr was the first person to have his name on three Supreme Court rulings.