The Celebrity Apprentice star was originally thought to be supporting Newt Gingrich for the political office, but stood behind Romney in an announcement at the Trump International Hotel & Tower in Las Vegas.
While Trump previously criticised many of Romney's proposed policies and ideals, he has since revealed that the pair shared "numerous meetings" in recent months which helped him come to a final decision.
Trump also insisted that his past remarks about Romney were not based on fact but rather lack of knowledge, saying: "I never knew him. I knew of him and respected him, but I really got to know him over the past few months."
Romney has said that he is "honoured" to have received Trump's endorsement and now hopes to also win the vote of Nevada citizens in the upcoming caucuses.
Last year, Trump hinted that he might run for the presidency in this year's election as part of the Republican Party, though he later ruled out a bid.
Trump dropped his membership of the Republican Party in December, following the cancellation of a debate he was to moderate.
He has repeatedly attacked President Barack Obama, claiming that the US is "going to hell" under his leadership.