Trump announced his support for the former Massachusetts governor at his Las Vegas hotel. And it was a Vegas show unlike any other with Trump taking the stage with Romney and his wife Ann.
"It's my honor, real honor, and privilege to endorse Mitt Romney," said Trump.
The reality show host and real estate mogul said he made his decision to endorse Romney after he got to know the presidential hopeful over the past few months- praising him as tough, smart and sharp.
"He's not going to allow bad things to continue to happen to this country," said Trump. "Gov. Romney, go out and get them."
Romney, who said he was honored to receive the endorsement, is hoping to win the endorsement of Nevada voters as the state holds its presidential caucuses on Saturday. He promised to turn Nevada's economy around.
"This is a very tough time for Nevada and I want to do everything in my power to get this economy going again so people can be in homes they can afford, and come here for tourism," said Romney.
Romney is the favored candidate in Nevada- a state he won when he sought the GOP nomination in 2008.
Meanwhile the endorsement was a surprising twist after Newt Gingrich's camp, which had been so confident of winning the real estate mogul's backing, leaked word that Trump would support the former House speaker.
"I'm just watching in amazement," said Gingrich, alluding to Trump. "Some people have this knack of gathering attention."
Trump said Romney's performances in presidential debates and his tough stance on China were reasons for giving him his endorsement.
And although he has said negative things about Romney in the past, including boasting he's a better businessman than Romney and has a higher net worth, he argued his comments were a reflection of not knowing the GOP presidential hopeful well enough.
Trump also said he would not run for president as an Independent if Romney becomes the GOP nominee.
The Associated Press contributed to this report.