Christopher Chaney, 35, 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 pocket.
Sentencing was scheduled for July 23.
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 their email account, he also went through their contact lists to find email 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 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 confessed to his scheme.
"I don't understand why any person who is rational would continue in that kind of behavior," Otero opined.
As part of the plea agreement, Chaney could pay a fine of up to $2.2 million. He must pay restitution to his victims, ranging from $15,000 to $400,000.
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. Aguilera's publicist Nicole Perez-Krueger declined comment. Messages left for representatives of Kunis and Johansson were not immediately returned.