José Manuel García Rodríguez was arrested in July 2005 in Carcarañá, Argentina, where he was known to inhabitants as "el Gordo España" (the fat Spaniard).
On Friday he arrived at Madrid's Barajas Airport on a flight from Buenos Aires, accompanied by two Interpol officers.
"García Rodríguez is accused of being Spain's biggest ever hacker, and could face a sentence of up to 40 years in prison if the charges against him are proven," said Graham Cluley, senior technology consultant for Sophos. "It's encouraging to see international authorities working closely together across continents to bring computer criminals to justice. If found guilty, el Gordo España could soon be on a prison diet of bread and water."
The 24-year-old García Rodríguez, said to have used the pseudonym "Tasmania" in hacking circles, fled Spain two years ago after being accused of stealing online banking passwords and breaking into accounts. Nine international arrest warrants have been issued for his detention.
"Criminals are stealing fortunes from online bank accounts, and all internet users need to be properly protected to ensure that their confidential passwords and usernames are not falling into the hands of hackers," said Cluley.