"I'm sorry for it," Bolton told US District Judge Theodore D Chuang during the hearing.
Reuters previously reported that Bolton would plead guilty under a deal with prosecutors that included a sentencing range from no prison time to as many as five years behind bars, with the final sentence to be determined by a judge.
As part of the agreement, Bolton agreed to pay a $US2.25 million ($A3.26 million) fine.
Bolton, 77, must make half that payment within five days of sentencing and the full payment within 90 days of sentencing.
He also committed to up to 100 hours of community service and to meet with intelligence and Justice Department officials for a debriefing.
Bolton will also forfeit his government pension.
Chuang scheduled sentencing for October.
Bolton is accused of sharing sensitive information with two relatives for possible use in a memoir he was writing, including notes on intelligence briefings and meetings with senior government officials and foreign leaders.
He pleaded not guilty to 18 criminal charges last year.
The book detailed Bolton's tenure as Trump's national security advisor during his first term.
In the book, Bolton described the president as unfit for office, sparking a public feud.
But prosecutors said on Friday that no classified information was published in Bolton's book The Room Where It Happened.
Authorities said Bolton's personal email was hacked by someone believed to be linked to Iran, which prosecutors reiterated on Friday.
The investigation into Bolton began before Trump returned to office in 2025 and had the backing of career federal prosecutors under president Joe Biden.
