Christie, at a news conference in Hanover, N.H., described the endorsement as an "easy decision." He cited Romney's experience in the private and public sectors, saying he "brings the best of both" to the job.
"America cannot survive another four years of Barack Obama, and Mitt Romney's the man we need to lead America and we need him now," Christie said.
The New Jersey governor accused the sitting president of trying to divide Americans by class, but said Romney believes "that the American pie can grow bigger."
The announcement came just hours before the Republican candidates were set to gather for a debate nearby at Dartmouth College. In securing the support of Christie, Romney will have at his side a tough-talking governor who during his two years in office has built a reputation as a fiscal hawk not shy about taking on the public employee unions.
The former Massachusetts governor is enlisting Christie at a time when his front-runner status is once again being challenged. Romney saw his lead slip after Texas Gov. Rick Perry jumped into the race in August. Perry has since fallen back in the polls, but businessman Herman Cain broke through to within a few points of Romney in several recent national polls.
Marking a fast turnaround for Romney, the new endorsement comes one week to the day since Christie called a news conference in Trenton to end once and for all the speculation that he would make a late entrance into the race.
Christie for months had said he would not run but then acknowledged that encouragement from others had him rethinking the decision in recent weeks. Even so, he said he never changed his mind, and he determined he had too much unfinished business to take care of in his home state.
The endorsement of Romney should leave no doubt that Christie is out as a potential candidate -- but in as a high-profile surrogate for the former Massachusetts governor.
Romney also has secured the endorsement of former Minnesota Gov. Tim Pawlenty, who bowed out of the race in August.
The Perry campaign, in a written statement Tuesday, suggested it was unfazed by the Christie announcement.
"Governor Perry has the utmost respect for Gov. Christie and looks forward to his help unseating President Obama next year," spokesman Ray Sullivan said. "Until then, Governor Perry will continue traveling the country talking about job creation and getting America working again. Rick Perry is the only candidate with a proven record of job creation, restraining spending, and lowering taxes to encourage strong economic growth and opportunity."