Mark Madoff, 46, was found hanged in his apartment in Manhattan's fashionable SoHo section, according to the official. A family member notified police around 7:30 a.m.
The official was not authorized to speak publicly about the case and spoke to The Associated Press on the condition of anonymity.
Mark Madoff and his brother, Andrew, were under investigation but hadn't faced any criminal charges in the massive Ponzi scheme that led to their father's jailing.
Bernard Madoff swindled a long list of investors out of billions of dollars and is serving a 150-year prison term in North Carolina. He was arrested on Dec. 11, 2008, after confessing his crimes to his family.
Madoff's sons, according to the family's attorneys, were the ones who turned him in.
The scandal put a harsh light on members of the family. The financier's brother, Peter, played a prominent role in the family's company. Mark and Andrew Madoff both worked on a trading desk at the firm, on a side of the business that wasn't directly involved in the Ponzi scheme.
A year ago, the court-appointed trustee trying to unravel Madoff's financial affairs sued several relatives, including Peter, Mark and Andrew, accusing them of failing to detect the fraud while living lavish lifestyles financed with the family's ill-gotten fortune.
The lawsuit accused Mark Madoff of using $66 million he received improperly to buy luxury homes in New York City, Nantucket and Connecticut.
In February, Mark Madoff's wife petitioned a court to change her last name and the last names of her children, saying her family had gotten threats and was humiliated by the scandal.
A call to Mark Madoff's attorney and his father's attorney was not immediately returned Saturday. Calls to the FBI and U.S. Attorney's office were also not immediately returned. Previously, spokespeople for the brothers had repeatedly denied that they had any knowledge of their father's crimes.
Police investigators were at Madoff's apartment Saturday morning, along with officials from the medical examiner's office, which will determine the cause of death.