The former New England Patriots tight end was jailed in 2013 for the murder of Odin Lloyd, a semi-professional player.
Lloyd was found near Hernandez's home in June 2013 having been shot six times.
Hernandez was later charged with first-degree murder and sentenced to life without parole.
Last week, he was found not guilty of a separate killing.
Daniel de Abreu and Safiro Furtado were killed in a drive-by in Boston, but Hernandez was acquitted.
The prosecution alleged Hernandez shot at the pair after his drink was spilled in a club, showing ‘disrespect’ to him.
Former NFL star Aaron Hernandez was jailed in 2013
Aaron Hernandez, once of the New England Patriots, has been found dead
The former star was found not guilty after 37 hours but was then convicted of a single charge: unlawful possession of a gun.
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 27-year-old was found by officers at the Souza Baranowski Correctional Center in Shirley, Massachusetts on Wednesday morning.
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Aaron Hernandez #81 of the New England Patriots
Aaron Hernandez was discovered hanged in his cell by corrections officers at the Souza Baranowski Correctional Center in Shirley
Massachusetts Department of Corrections
A statement from the Massachusetts Department of Corrections said: "On April 19, 2017 Aaron Hernandez was discovered hanged in his cell by corrections officers at the Souza Baranowski Correctional Center in Shirley, Massachusetts at approximately 3:05am.
"Lifesaving techniques were attempted on Mr. Hernandez and he was transported to UMASS Leoinster where he was pronounced dead at 4:07am by a physician at the hospital.
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"Mr Hernandez was in a single cell in general population housing unit. Mr Hernandez hanged himself utilizing a bed sheet that he attached to his cell window. Mr Hernandez also attempted to block his door from the inside by jamming the door with various items.
"The Massachusetts State Police are on scene and the investigation continues. Mr Hernandez's next of kin have been notified."
Aaron Hernandez was jailed for life without parole in 2013
Hernandez has a four-year-old daughter with Shayanna Jenkins-Hernandez.
The pair are not married but Miss Jenkins recently took her boyfriend's name.