The 89-year-old Democrat, the only World War II veteran in the upper house, told The (Newark) Star-Ledger he plans to remain active until he leaves office in January 2015.
"I am not announcing the end of anything. I am announcing the beginning of a two-year mission to pass new gun safety laws, protect children from toxic chemicals and create more opportunities for working families in New Jersey," Lautenberg said. "While I may not be seeking re-election, there is plenty of work to do before the end of this term and I'm going to keep fighting as hard as ever for the people of New Jersey in the U.S. Senate."
Lautenberg faced a primary challenge from Cory Booker, with early polls giving the Newark mayor a substantial lead. Booker has gained a national reputation as the child of a black middle-class family who grew up in the suburbs, went to Ivy League schools and then decided to live and run for office in a troubled city.
While Lautenberg's staff reacted to Booker's candidacy by calling him "disrespectful" and the senator said Booker needed "a spanking," Booker praised Lautenberg on Thursday.
"Sen. Frank Lautenberg has been a champion for the people of New Jersey for decades and his legacy of service will improve the lives of all Americans for years to come," Booker said in a statement.
In the Senate, Lautenberg has supported Amtrak and pushed for laws limiting smoking.
Born in Paterson, the son of a textile worker, Lautenberg served in the Army and then went to Columbia University on the G.I. Bill. He built a successful computer payroll business, Automatic Data Processing.
Lautenberg ran for the Senate against Rep. Millicent Fenwick, R-N.J., in 1982. He won with 51.5 percent of the vote. Ironically, the 72-year-old Fenwick's age became a campaign issue.
In 2000, Lautenberg retired, at least partly because of problems with his colleague, Sen. Robert Torricelli, D-N.J. When Torricellli dropped out of his race for a second term in 2002 because of a campaign-financing scandal, Lautenberg stepped in five weeks before the election and won.