Corzine heads the Democratic Senatorial Campaign Committee and previously declined to make an endorsement.
Kerry is the strongest national candidate and the one most likely to beat President Bush, Corzine said at a news conference Tuesday.
"John Kerry has proven with his life that he can represent all Americans," Corzine said.
The first-term senator promised he would help Kerry raise money. The endorsement also would allow others to back Kerry financially, Corzine said.
Kerry supporters in New Jersey said Corzine's announcement would elevate the candidate above others in a crowded field.
Last week Kerry came in first in the Iowa caucuses and led most polls going into Tuesday's primary in New Hampshire.
"To have the chair of the Democratic Senatorial Campaign Committee join our efforts is a huge coup. This has major national implications, and provides a big boost for us here in New Jersey," said U.S. Rep. Bill Pascrell, co-leader of the Kerry for President campaign in New Jersey.
Last month, many key New Jersey Democrats decided to back former Vermont Gov. Howard Dean (search). Gov. James E. McGreevey (search) led the push for Dean, and became the first sitting governor to back him.
In a December interview with The Associated Press, Corzine declined to back any one candidate, saying he had three Senate colleagues running and to choose would be inappropriate.
Corzine did question Dean's credentials.
"There will be some concerns about fleshing out his national security positions," Corzine said in the AP interview. "Some will be concerned about his attitude, which is not fleshed out completely, about laying down initiatives on re-regulating the economy."
However, Corzine said that conventional wisdom in political circles was that "barring some great faux pas," Dean would win the nomination.
On Tuesday, Corzine said Kerry's positions on the economy, education and homeland security won him over.
"He is an incredible fighters for the issues that I and the people of New Jersey care about deeply, health care, education, the environment and the economy," Corzine said.
Corzine, 56, shattered spending records in his first run for Senate, shelling out $60 million of his own money to win in 2000. Since then, Corzine has grown to become a leader in Democratic politics in New Jersey and nationally.
As chairman of the Democratic Senatorial Campaign Committee, Corzine leads his party's recruitment and fund-raising efforts for the 2004 Senate elections.