Boston Marathon bomber Dzhokhar Tsarnaev has been sentenced to die by lethal injection for the 2013 terror attack.

The US federal jury in Massachusetts reached the decision today after more than 14 hours of deliberations over three days.

The 21-year-old 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.

Tsarnaev showed no reaction as the jury sentenced him.

His head was down slightly and his hands were folded in front of him as the sentence was handed down.

The defence asked that the jury be polled, and each confirmed that the verdict represented his or her decision.