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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A friend of the accused Boston Marathon bomber has been found guilty of lying to US authorities in a terrorism investigation. Robel Phillipos, 21, was charged with lying about having visited suspected bomber Dzhokhar Tsarnaev's dorm room three days after the attack, which killed three people and injured more than 260.
Phillipos and two friends removed a backpack containing empty fireworks shells from the suspected bomber's room. Phillipos, of Cambridge, Massachusetts, was on trial for two criminal counts of lying to investigators, one for saying he did not remember the visit and one for denying it.
He later signed a written confession admitting to it, following an FBI interrogation.
The US Department of Justice has said that the jury has reached a verdict in the trial of the college friend of accused Boston Marathon bomber, who was charged with lying to authorities in a terrorism investigation.
Robel Phillipos, 21, was charged with lying about having visited suspected bomber Dzhokhar Tsarnaev's dorm room three days after the attack, which killed three people and injured more than 260.
A US judge has has overturned a bid by the man accused of the Boston Marathon bombings to move his trial out of the city where the atrocity took place.
However, US District Judge George O'Toole agreed to delay the start of the trial of Dzhokhar Tsarnaev by two months to January, pushing it back to after the US holiday season.
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