"I am here today to plead guilty to conspiracy for which I am deeply ashamed and sorry," he told federal Judge Laura Taylor Swain, who set bail at $5 million, with $1 million cash, the New York Post reported.
Peter Madoff, 66, was arrested by FBI agents Friday morning. The New York Times said he had reached a plea deal with prosecutors on charges of falsifying documents, filing false tax returns and lying to regulators in exchange for a 10-year prison term.
In U.S. District Court in Manhattan, he pleaded guilty to two counts of conspiracy and falsifying records and was expected to be sentenced Oct. 4, the Post said.
As part of the plea agreement he also agreed to forfeit $143 million, which was believed to exceed all of his assets, the Times said.
"My brother is Bernard Madoff," he told the court. "At no time before 2008 did I know anyone … was committing a Ponzi scheme. I encouraged my own family to invest. My wife lost millions of dollars."
Bernard Madoff, 74, began serving a 150-year sentence at a federal prison in North Carolina in 2009 and has said he acted alone.
Peter Madoff, a securities lawyer, held numerous positions in Bernard L. Madoff Investment Securities, ranging from chief compliance office to general counsel. He was a salaried employee for the family-run company for 40 years and never became a partner.
He is the eighth of 13 defendants to plead guilty in the case.