Azamat Tazhayakov, a baby-faced 20-year-old, put his hands over his face and shook his head as guilty verdicts were read on charges of obstruction of justice and conspiracy.
It was the first trial stemming from the twin bombings, which killed three and injured more than 260 near the marathon's finish line in April last year. His mother sobbed loudly and rocked in her seat.
The jury found Tazhayakov conspired with friend Dias Kadyrbayev to take from Tsarnaev's room a backpack containing fireworks that had been emptied of their explosive powder. Prosecutors said the explosive powder could have been used to make bombs.
Tazhayakov faces a maximum 20-year prison sentence for obstruction and a five-year maximum for conspiracy when he is sentenced in October.