Members of the federal jury cried but Dzhokar Tsarnaev remained reactionless as the death sentence was read out in a federal court in Massachusets.
Tsarnaev - wearing a dark sport coat with a light-coloured shirt - had his head bowed slightly, with his hands folded in front of him.
Following the announcement, Judge George O'Toole described the jury as a "model for future juries".
"You have sat through a trial that has had horrible images and poignant testimony," he added.
Boston bomber Dzhokhar Tsarnaev has been sentenced to death but it could be many years before that actually happens.
ITV News Washington Correspondent Robert Moore said there will be many appeals before he is executed.
The appeal process, mandatory in this case, will last many years. If it fails, Tsarnaev will be put to death at federal facility in Indiana.
The jury in the trial of Dzhokhar Tsarnaev took 14 hours of deliberations to reach their decision to sentence the 21-year-old to death by lethal injection.
Tsarnaev was convicted last month of all 30 federal charges against him, 17 of which carried the possibility of the death penalty.
The bombing killed three people and injured more than 260 others.
Tsarnaev killed a Massachusetts Institute of Technology police officer days later.
The defence sought to save Tsarnaev's life by pinning most of the blame on his radicalised older brother.
But prosecutors portrayed Tsarnaev as an equal partner in the attack and so heartless he placed a bomb behind children, killing an eight-year-old boy.
In response to the sentencing to death of Dzhokar Tsarnaev, the US Attorney General said: "The ultimate penalty is a fitting punishment for this horrific crime."
Dzhokar Tsarnaev coldly and callously perpetrated a terrorist attack that injured hundreds of Americans and ultimately took the lives of three individuals: Krystle Marie Campbell, a 29-year-old native of Medford; Lingzi Lu, a 23-year-old Boston University graduate student from China; and Martin Richard, an 8-year-old boy from Dorchester who was watching the marathon with his family just a few feet from the second bomb.
Dzokhar Tsarnaev will be killed by lethal injection after being sentenced to the death penalty by a jury in Boston.
As ITV News Washington Correspondent Robert Moore reports, Tsarnaev's brother Tamerlan was killed by police in a shootout shortly after the deadly attack on the city's marathon in 2013.
Death penalty for Tsarnaev. One brother died in a police shootout 2 years ago. Now the other faces lethal injection in Indiana. #Tsarnaev
Tsarnaev was reported as emotionless in his seat as the verdict was read out.
Dzhokhar Tsarnaev, the Boston marathon bomber, has been sentenced to the death penalty.
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