Miami police said they found Villareal's body in his apartment Saturday, but did not disclose a cause of death.
Villareal, an economist and Christian Liberation Movement activist, was freed in 2010 and immediately flew from a Havana prison to Spain. He moved to Miami several months later and was under psychiatric care for the "inhumane treatment" he received in prison, he said in 2010.
"The years of isolation have left their mark on all of us. When my wounds have healed, someone should pay for what I have suffered," he told a Spanish newspaper in 2010.
Other dissidents reported Villareal began suffering from psychiatric problems after he was sentenced to 15 years imprisonment for his activism, the Miami newspaper El Nuevo Herald said Tuesday.