Medics attempted but failed to resuscitate the 75-year-old, according to the Catalan justice department.
It went on to say that “everything indicates” McAfee committed suicide.
His firm developed the first commercial anti-virus software, making him a controversial figure in the computer industry.
McAfee VirusScan was instrumental in launching a multibillion-dollar computer business, and it was subsequently purchased by Intel for more than $7.6 billion (£4.7 billion).
Despite making millions through consultancy work, speaking engagements, crypto-currencies, and selling the rights to his life narrative, John McAfee was detained in Spain in October 2020 as he was ready to board a plane to Turkey, and accused of failing to submit tax returns for four years.
McAfee allegedly avoided tax obligation by having his income transferred into bank accounts and cryptocurrency exchange accounts in the names of nominees, according to the US Justice Department.
He was also accused of hiding assets in the names of others, including a boat and real estate property.
On Wednesday morning, Spain’s National Court approved his extradition to the United States to face the accusations.
McAfee has alleged for years that there was a plan to assassinate him, but the court ruled there was “no illuminating proof” that he was being pursued for political or ideological purposes, according to Spanish daily El Pais.
In a hearing earlier this month, he claimed that the charges against him were politically motivated and that if he were sent to the United States, he would spend the rest of his life in jail.
McAfee’s lawyer, Nishay Sanan, said he planned to defend all of the accusations.
“The US government is attempting to destroy John McAfee once again,” Mr Sanan told NPR on Wednesday. “This was a man who was not afraid to fight. And he will always be a warrior in the hearts of those who knew him.”
The entrepreneur, who was born in Gloucestershire, England, rose to popularity in the 1980s after launching his software firm and releasing McAfee VirusScan.
Despite being a computer security pioneer, he acknowledged to the BBC that he never used the program – or any anti-virus software for that matter – on his personal machines.
In 2013, he told the BBC’s technology correspondent Leo Kelion, “I defend myself by continuously changing my IP [internet protocol] address, not associating my identity to whatever device I use, and not logging on to sites where you may pick up a virus.”
“For example, I don’t visit pornographic websites.”
He also ran for President of the Libertarian Party in the 2016 and 2020 presidential elections, but was unsuccessful.
McAfee despises taxes, stating in 2019 that he hasn’t filed tax returns in eight years because “taxation is unlawful.”
He was temporarily imprisoned in the Dominican Republic in the same year on suspicion of smuggling firearms into the nation.