Mr Christie pulled out of the Republican White House race after a dismal showing in the nomination contests in Iowa and New Hampshire, where he finished 10th and sixth respectively.
He said Mr Trump was the party’s best hope of winning back the presidency from the Democrats.
“The best person to beat Hillary Clinton in November … is undoubtedly Donald Trump,” he said.
“I will lend my support between now and November every way that I can for Donald to make the campaign an even better campaign than it’s already been and do what he needs to do to help to make the country everything we want it to be for our children and grandchildren.”
Mr Trump welcomed the move, saying: “This is the one endorsement that I felt very strongly about I wanted to get.”
Mr Christie, 53, a former federal prosecutor, is a moderate Republican running a largely Democratic state and was once seen as one of the party’s best hopes in 2016.
His endorsement of Mr Trump — an outsider whose brash, provocative style of politics has upended the Republican race — marks a potential turning point in the campaign.
Mr Trump has won three straight nominating contests in New Hampshire, South Carolina and Nevada, convincing some mainstream Republicans that he may have too much momentum to be stopped, especially if he wins big in the key southern US primaries four days away.
Mr Trump has vowed to build a wall on the US-Mexican border to fight illegal immigration, called for a temporary ban on Muslims entering the US and promised to take a tough stance on trade against China.
Republican rivals Marco Rubio and Ted Cruz ganged up on Mr Trump at the raucous debate, in a last-ditch bid to keep the billionaire from winning in states holding nominating contests on Super Tuesday that could set him up to clinch the presidential nomination.
“A con artist is about to take over the Republican Party and the conservative movement and we have to put a stop to him,” Mr Rubio warned.
But they only dented Mr Trump’s momentum, according to opinion polls and online betting markets.
A good showing on Tuesday would put Mr Trump within touching distance of having enough delegates to secure the party nomination in July.
That has sparked a virtual civil war among Republicans, between those who believe a Trump victory would destroy the party and those who believe he could beat Mrs Clinton.
This week, 2012 Republican candidate Mitt Romney joined in the charge against Mr Trump and begged campaigns to enjoin the fight.
Millions have already been channelled into anti-Trump adds, and into lambasting the real estate mogul on Twitter and in other media.
Mitt Romney joined the fray by repeatedly calling on Mr Trump to release his tax returns, suggesting there may be a “bombshell” that would make him unelectable.
Mrs Clinton is battling US senator Bernie Sanders of Vermont for the Democratic nomination.
Al-anwar Newspaper English news in Australia
