Christie's resignation has fueled speculation that he will run for governor, the Philadelphia Inquirer reported.
Christie, a Republican, was appointed seven years ago by President George W. Bush. He resigned in a letter to U.S. Attorney General Michael Mukasey, the newspaper reported.
Among Republicans, Christie is seen as a possible contender to take on Democratic Gov. John Corzine.
No Republican has won a statewide office in New Jersey since former Gov. Christie Whitman was re-elected in 1997.