He had hung himself with a bed sheet that he attached to a cell window.
The downfall of Aaron Hernandez is one of the most upsetting stories to come out of professional sports, the unraveling of a young man with so much promise who committed a horrific crime, then took his own life.
The attorney is calling for a "transparent and thorough investigation" into the death of his client.
Authorities tried to revive the former New England Patriots tight end, and he was pronounced dead at UMass Memorial - HealthAlliance Hospital in Leominster at 4:07am.
Hernandez's two murder trials revealed a troubled man who often used illegal narcotics and at times believed strangers were challenging or disrespecting him.
Baez - who successfully got Hernandez acquitted of double murder charges in Boston and was planning to represent him in his appeal of the Odin Lloyd case - said in a statement obtained by several Boston media outlets that his firm would be conducting a private investigation into the death.
While serving the life sentence in a MA prison, double homicide charges were also brought against him for the 2012 murders of Daniel Abreu, 29, and Safiro Furtado, 28. According to the Massachusetts Department of Corrections, Hernandez's death is a cut and dry suicide.
At the height of his career with the New England Patriots, now Super Bowl champions, Hernandez was on a five-year contract worth $40m (£31m).More news: Alec Baldwin: Kendall Jenner's unlikely defender
The Worcester County district attorney's office and the Correction Department are investigating.
He immediately proved himself a major asset to the Patriots, catching six passes for 101 yards in a game against the New York Jets in Week 2 of the 2010 season.
"This is a shocking and sad end to a very tragic series of events that has negatively impacted a number of families".
Team spokesman Stacey James said the Patriots were aware of the reports of Hernandez's death but did not anticipate the club commenting. He was just 27 years old. He says Hernandez would have been transferred to a mental health unit if there was any concern about his well-being.
Hernandez was acquitted Friday in the 2012 double murder that prosecutors said was fuelled by his anger over a spilled drink.
Hernandez was in a single cell in a general population housing unit at the prison.
MA prison officials say former National Football League star Aaron Hernandez has hanged himself in his cell and has been pronounced dead at a hospital.