Born in England's Gloucestershire in 1945 as John David McAfee, he started McAfee Associates in 1987 and led an eccentric life after selling his stake in the antivirus software company named after him in the early 1990s.
A few hours earlier, Spain's National Court had approved McAfee's extradition to face tax-related criminal charges in the U.S.
An investigation into his death is underway, but the report states that all signs point to suicide.
News of his death came shortly after a Spanish court had agreed on Wednesday to extradite McAfee, who faced charges of tax-evasion in the United States. McAfee was arrested last October and has long maintained that the tax evasion charges were motivated by politics.
McAfee still had opportunities to appeal his conviction but could not stand more time in jail, the lawyer Javier Villalba said.
The U.S. calculates that McAfee owed more than US$4.2 million in taxes, the documents show.
Sources with knowledge of the development told local outlet El Periodico that McAfee had committed suicide, and that prison staff attempted to resuscitate McAfee without success. "I have nothing. Yet, I regret nothing", he said.
Janice McAfee said in a post on Twitter on Sunday, Father's Day: "Now the USA authorities are determined to have John die in prison to make an example of him for speaking out against the corruption within their government agencies".
He was accused of killing his neighbor in Belize in 2012, but no charges were ever filed.
In the June 9 post on Twitter, McAfee noted that "after uncountable lawsuits and the reach of the FED's I now have nothing. The things you believe you own, in reality own you". According to federal investigators, he allegedly bought the touted currencies for cheap, and then sold them off at inflated prices, due to his Twitter followers following his investment advice. "Every day I wake up with him", he told Wired.