Paul Konigsberg, 78, pleaded guilty under an agreement that requires him to cooperate with investigators and to forfeit $4.4 million. Prosecutors agreed that he was unaware that Madoff's investment business was a house of cards, but said he did know that he was using backdated trading records when he prepared tax returns.
Konigsberg told the judge he wanted to take responsibility for his actions.
"It is important for me, your honor, to let you know that I was not aware of Madoff's horrific and evil Ponzi scheme," he added.
Madoff, 76, who pleaded guilty in 2009, is serving a 150-year federal prison sentence. Many of his former employees have been convicted or pleaded guilty, including his brother, Peter, and one of his sons hanged himself two years after his father was arrested.
The fraud was one of the largest and longest-running in U.S. financial history. Madoff claimed to have a system that guaranteed investors a steady return of 10 to 12 percent a year while actually using newly invested money to make payments.
Prosecutors did not say what kind of cooperation they expect from Konigsberg and whether the investigation is still active.