He was being held on Ward One at Brians 2 Prison, a 40 minute drive north-west of Barcelona.
Prison guards attempted to revive him but were unable to save his life.
Hours before the news broke, Spain’s Audiencia Nacional court had agreed his extradition for alleged tax evasion.
Routine investigations, one led by a local court and another by prison authorities, are underway.
McAfee was born on an American air base in Gloucestershire to a British mum and American dad.
He is believed to have spent a large part of his time ahead of his arrest on October 3 in the Catalan province of Tarragona.
Local reports soon after his detention pointed to him bunkering down at a disused hotel in the coastal resort of Cambrils which was going to start up as a brothel but ended up being used as a bitcoin farm.
He is said to have been travelling on a British passport when he was arrested, although police never confirmed the reports.
He was remanded to the jail where he was found dead on Wednesday afternoon days after appearing before a duty judge by video link.
Around the time of his arrest last year in Spain, the US Justice Department announced charges shortly after the Securities and Exchange Commission (SEC) revealed it had brought civil charges against McAfee, alleging he made millions in undisclosed compensation from misleading cryptocurrency offerings on Twitter.
He was alleged to have earned millions from promoting cryptocurrencies, as well as through consulting work, speaking engagements, and selling the rights to his life story for a documentary.
Despite this income, the US Justice Department accused him of failing to file any tax returns from 2014 to 2018.
According to court papers, McAfee allegedly registered a number of purchases under someone else’s name, including a yacht, car and property that he bought, in a bid to avoid paying taxes.
A judge at Spain’s Audiencia Nacional court agreed to extradite him to the States to face trial hours before he was found dead for tax evasion over the three years between 2016 and 2018.
McAfee had claimed he was the victim of political persecution after complaining about tax corruption.
The tech guru famously went on a naked shooting spree in September 2017 because he thought intruders from Belize were trying to break into his home in Tennessee.
He had previously lived in Belize and his home there was raided by the Belize Police Gang Suppression Unit because he was suspected of cooking meth.
However, no drugs were found and he was never charged.