The 12-person jury took a day and a half to find the 21-year-old guilty, using a 32-page verdict form of 30 counts related to the attacks, the murder of a police officer, a car jacking and shootout while on the run.
The Muslim immigrant of Chechen descent, who took US citizenship in 2012, now faces the death penalty or life in prison without parole when he is sentenced by the same jury after a later second phase of the trial.
Three people were killed and 264 others wounded, including 17 who lost limbs, in the twin blasts at the city's marathon on April 15, 2013.
Tsarnaev was arrested four days later, hiding and injured in a boat on which he had scrawled a bloody message apparently justifying the attacks to avenge the US wars in Iraq and Afghanistan.
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The marathon bombings shocked the relatively small northeastern city of Boston and revived fears of terrorism in the United States after the September 11, 2001 attacks on New York, Washington and Pennsylvania.
Tsarnaev's lawyers admitted that he planted one of the pressure-cooker bombs concealed in a backpack, but portrayed him as a feckless accomplice, bullied or manipulated into taking part by his more radical elder brother.
The defendant looked pale as he entered court to hear the verdict dressed in a blue sweater and dark blazer. His lawyer Judy Clarke gave him a brief pat on the back as he sat down.
Seventeen of the charges -- one to 10 and 12 to 18 -- carry the possibility of the death penalty.
Prosecutors spent four weeks building their case, calling 92 witnesses in an effort to paint Tsarnaev as an active and willing bomber alongside his elder brother, who was killed by police while on the run.