Christopher Chaney, 35, of Jacksonville, Fla., entered his plea in federal court to nine counts, including unauthorized access to a computer and wiretapping. He was immediately taken into custody with his head hung low and his hands in his pockets.
Sentencing was scheduled for July 23, and Chaney could face up to 60 years in prison. Under a plea bargain, he also could pay a fine of up to $2.2 million and must pay restitution to his victims, ranging from $15,000 to $400,000.
Chaney was arrested in October as part of a yearlong investigation of celebrity hacking that authorities dubbed "Operation Hackerazzi."
Prosecutors said Chaney illegally accessed the email accounts of more than 50 people in the entertainment industry between November 2010 and October 2011.
Nude photos Johansson had taken of herself were later posted on the Internet. Johansson told Vanity Fair for its December issue that the photos were meant for Ryan Reynolds, now her ex-husband.
Chaney mined through publicly available data to figure out password and security questions for celebrity accounts. Once he had control of an email account, he also went through contact lists to find accounts of fellow stars.
He also hijacked a forwarding feature so a copy of every email a celebrity received was sent to an account he controlled, according to court documents.
A search warrant unsealed and obtained by The Associated Press said Chaney's computer hard drive contained numerous private celebrity photos and a document that compiled their extensive personal data.
In one instance, Chaney posed as stylist Simone Harouche and sent an email from her account to Aguilera -- one of Harouche's clients -- asking the singer for scantily clad photographs, prosecutors said. Some of Aguilera's photos appeared online.
Chaney forwarded many of the photographs to two gossip websites and another hacker, but there wasn't any evidence that he profited from his scheme, authorities said. He has since apologized for his actions.
In ordering Chaney to be held behind bars until he's sentenced, U.S. District Judge S. James Otero noted the defendant continued to hack into celebrity email accounts -- namely one belonging to a star only referenced by initials C.B. -- after he was interviewed by FBI agents and acknowledged the scheme.
"I don't understand why any person who is rational would continue in that kind of behavior," Otero opined.
It's unclear whether Aguilera, Kunis or Johansson would write letters to the judge or attend Chaney's sentencing, but all three women agreed to have their identities made public after an indictment was announced last October.
"I have confidence that justice will prevail and that the court will set a precedent for a `no tolerance policy' in regards to identity theft, computer hacking and invasion of privacy," Johansson said in a statement to the AP.
Aguilera's publicist Nicole Perez-Krueger declined comment. An email messages left for a Kunis representative was not immediately returned.
Another celebrity, ESPN reporter Erin Andrews, attended the Los Angeles sentencing of insurance executive Michael Barrett in March 2010 after he pleaded guilty to interstate stalking for secretly shooting nude videos of her through hotel peepholes. Andrews urged a harsh sentence, but Barrett agreed to serve a 27-month term.