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.
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