Vladimir Drinkman, who was extradited from the Netherlands, said he didn’t conspire with other people to hack into major financial networks and sell data to other crooks.
Reuters said the attacks, which started in 2005, meant 160 million credit card numbers and hundreds of millions of dollars were extracted from corporations and individual people.
Networks hacked included a Visa licensee, 7-Eleven, JC Penney and Carrefour.
He faces a trial in Newark at the end of his April and if convicted could spend 30 years in jail.
He fought against extradition after being arrested in 2012. Three alleged co-conspirators have so far not been caught, while a fourth Dmitriy Smilianets was also extradited from the Netherlands and is in jail in the USA.