The Department of Corrections said the 27-year-old was found hanged in his cell just after 3am local time on Wednesday morning.
“Life-saving techniques were attempted on Hernandez and he was transported to hospital where he was pronounced dead at 4.07am,” its statement added.
Massachusetts State Police has now launched an investigation into the suicide at the Souza Baranowski Correctional Centre in the town of Shirley.
In a statement, his lawyer said there had been no warning signs that Hernandez was planning to kill himself.
Jose Baez added: “There were no conversations or correspondence from Aaron to his family or legal team that would have indicated anything like this was possible. Aaron was looking forward to an opportunity for a second chance to prove his innocence.”
At the height of his career with the New England Patriots, currently Super Bowl champions, Hernandez was on a five-year contract worth $40m (£31m).
The tight end was dropped by the team just hours after he was arrested for the 2013 murder of semi-professional footballer Odin Lloyd, for which he was sentenced to life in prison without parole.
Less than a week ago, a jury had cleared Hernandez of killing Daniel de Abreu and Safiro Furtado in a 2012 drive-by shooting after a fight outside a nightclub in Boston’s South End.
Hernandez was acquitted on seven of the eight charges he had faced, and was only convicted on an illegal firearms charge.
Prosecutors had accused the former NFL star of shooting the men because his drink had been spilled inside the club.
Following the acquittal, Patriots head coach Bill Belichick had described Hernandez’s fall from grace as a “tragedy”.
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