A relative and the state medical examiner said his body was found on Friday in his Brooklyn, New York apartment. Authorities say he killed himself.
Swartz was facing hacking charges at the time of his death.
Prosecutors allege he was illegally downloading millions of academic papers from online archive JSTOR, which they say, he intended to distribute for free.
His federal trial was due to begin next month.
Swartz had an early start in computer programming and co-authored an early version of the RSS specification at age 14.
He was among the founders of the Demand Progress campaign group, which advocates against internet censorship.
Swartz's lawyer Elliot R. Peters confirmed the news the death in an email to the MIT university newspaper the Tech.
"The tragic and heartbreaking information you received is, regrettably, true," he wrote.