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BOSTON — Dzhokhar Tsarnaev sat stone-faced in a federal court here on Friday as a jury sentenced him to death for setting off bombs at the 2013 Boston Marathon that killed three people and injured hundreds more in the worst terrorist attack on American soil since Sept. 11, 2001.

The jury, which last month convicted Mr. Tsarnaev, 21, of all 30 charges against him, 17 of which carry the death penalty, took more than 14 hours to reach its decision.

The jury — seven women and five men — sentenced Mr. Tsarnaev to death on six of the 17 counts — all six related to his planting a pressure-cooker bomb on Boylston Street, which his lawyers never disputed.

In reaching its decision, the jury found that Mr. Tsarnaev had shown no remorse for his actions, and it rejected the defense argument that his older brother, Tamerlan, a self-radicalized jihadist, had brainwashed him into joining in the bombings.

It was the first time a federal jury had sentenced a terrorist to death in the post-Sept. 11 era, according to Kevin McNally, director of the Federal Death Penalty Resource Counsel Project, which coordinates the defense in capital punishment cases.

Mr. Tsarnaev stood, his hands folded in front of him, as the jurors made their way out of the courtroom. Moments earlier, the judge, George A. O’Toole Jr. of Federal District Court, thanked them and sent them on their way: “So jurors, that’s it. You are now discharged.”

For many in the Boston area, the sentencing brought “a small amount of closure to the survivors, families and all impacted by the violent and tragic events,” Mayor Martin J. Walsh said. “We will forever remember and honor those who lost their lives and were affected by those senseless acts of violence on our city.”

Attorney General Loretta E. Lynch called the death sentence a “fitting punishment.”

In Russia, when contacted by a reporter and informed of the verdict, Mr. Tsarnaev’s father, Anzor, simply exhaled and hung up. He then turned off his cellphone.

Prosecutors portrayed Mr. Tsarnaev, who immigrated to Cambridge, Mass., from the Russian Caucasus with his family in 2002, as a coldblooded, unrepentant jihadist who sought to kill innocent Americans in retaliation for the deaths of innocent Muslims in American-led wars in Iraq and Afghanistan.

The bombings transformed the marathon, a cherished rite of spring, from a sunny holiday on Boylston Street to a smoky battlefield scene, with shrapnel flying, bodies dismembered and blood saturating the sidewalks; three people were killed outright, while 17 people lost at least one leg. More than 240 others sustained serious injuries, some of them life-altering.

“After all of the carnage and fear and terror that he has caused, the right decision is clear,” a federal prosecutor, Steven Mellin, said in his closing argument. “The only sentence that will do justice in this case is a sentence of death.”

With death sentences, an appeal is all but inevitable, and the process generally takes years if not decades to play out. Of the 80 federal defendants sentenced to death since 1988, only three, including Timothy J. McVeigh, the Oklahoma City bomber, have been executed. Most cases are still tied up in appeal. In the rest, the sentences were vacated or the defendants died or committed suicide.

The Tsarnaev verdict goes against the grain in Massachusetts, which has no death penalty for state crimes and where polls showed that residents overwhelmingly favored life in prison for Mr. Tsarnaev. Many respondents said that life in prison for one so young would be a fate worse than death, and some worried that execution would make him a martyr.

But the jurors in his case had to be “death qualified” — that is, they all had to be willing to impose the death penalty to serve on the jury. So in that sense, the jury was not representative of the state.

Before they could decide that Mr. Tsarnaev should receive the death penalty, the jurors had to wade through a complicated, 24-page verdict slip. On it, they had to weigh the aggravating factors that would justify his death as well as the mitigating factors, presented by the defense, that would argue for him to live.

Despite that complicated process, Judge O’Toole told jurors that their final decision should not be based on a numerical comparison of aggravating factors to mitigating factors. Rather, he said, they should use their individual judgment and internal moral compass.

The final words that jurors heard before beginning deliberations on Wednesday were a blistering assessment from William Weinreb, the lead prosecutor, about why Mr. Tsarnaev should be executed and not simply locked up.

“The callousness and indifference that allows you to destroy people’s lives, to ignore their pain, to shrug off their heartbreak — that doesn’t go away just because you’re locked up in a prison cell,” Mr. Weinreb told them. “It’s what enables you to be a terrorist, and it’s what insulates you from feelings of remorse.”

Life imprisonment, he added, is the minimum punishment authorized by law for these deaths — the three at the marathon and the death a few days later of an M.I.T. police officer. It is “a lesser punishment than death,” he said, even though some argue that a lifetime in prison is worse.

“Does he deserve the minimum punishment, or do these crimes, these four deaths, demand something more?” Mr. Weinreb asked the jury. “Please ask yourself that question when you go back to deliberate.”

The verdict is a rare defeat for Judy Clarke, the lead defense lawyer and renowned opponent of the death penalty. Ms. Clarke has represented a number of notorious defendants, including Theodore J. Kaczynski, the Unabomber; Eric Rudolph, the Olympic Park bomber; and Jared L. Loughner, who killed six people in an assassination attempt on Representative Gabrielle Giffords.

Ms. Clarke’s expertise is in negotiating deals in which her clients plead guilty in exchange for sentences of life in prison. But in this case, the government was determined to get a death sentence and rejected her overtures.