Christie received a thundering reception as he took to the stage in front of thousands at a convention center in Fort Worth, Texas.
"I want Texas to be Trump country on Tuesday," Christie said.
Trump said he and Christie had decided to keep the endorsement secret until announcing it Friday and described it as "sort of a shocker." He said the surprise from reporters was clear when they walked into a pre-rally news conference to announce Christie's backing.
The announcement was indeed a stunner, and gives a big boost to Trump as he heads into next week's Super Tuesday primaries.
"Now maybe others will follow Christie's lead and say 'I can be a good Republican, I can be an establishment Republican, and still endorse Donald Trump,'" said Rider University Political Scientist Ben Dworkin.
Reaction among Christie's constituents was mixed.
"I have no idea why he would do it. I wish he hadn't. I think it's a mistake," said Dan Haggerty of Lawrenceville.
"I really think Donald Trump has a great chance to win and with Christie backing him they can go all the way," said Anthony Hearn of Bordentown.
BREAKING: NJ Gov. Chris Christie: "I am proud to be here to endorse Donald Trump for president of the United States? https://t.co/h4cstjEX7u— ABC News (@ABC) February 26, 2016
Also at the rally Friday, Christie slammed Sen. Marco Rubio.
Christie says, "We don't need any more of these Washington, D.C., acts."
Christie memorably attacked the freshman senator at the final Republican debate before the New Hampshire primary, tripping up Rubio in a moment that likely contributed to his poor performance in the state's primary. Christie accused him of parroting the same lines repeatedly and said a president should be able to think on his feet.
The endorsement from Christie comes the day after Rubio changed tactics in Thursday's GOP debate and began to attack Trump on a variety of fronts, from his business background to his preparedness to lead the nation. Rubio continued on Friday morning, calling Trump "a con artist" during a round of morning television interviews.
Trump says Rubio is a "desperate guy" and said, "I don't think he's of presidential caliber."