Trump was set to make the announcement in Las Vegas at the luxury hotel bearing his name, with Romney in attendance. Nevada holds its presidential caucuses on Saturday.
Speaking with reporters before the announcement, Trump said he had several meetings with Romney during the past several months and that those meetings helped influence his decision about an endorsement.
He also cited Romney's debate performances and tough stance on China as reasons.
Trump, who publicly had expressed less-than-enthusiastic support for Romney, said his past comments were a reflection of not knowing the former Massachusetts governor very well.
"I never knew him. I knew of him and respected him, but I really got to know him over the past few months," Trump said. "I've had numerous meetings with him."
Gingrich's camp was so confident of Trump's endorsement that those close to the former House speaker confirmed it Wednesday night for news organizations, including the AP. One of those officials said Trump had "sent signals" to Gingrich that he would support him. That individual declined Thursday to elaborate on what those signals were.
On a tour of a Las Vegas manufacturing facility Thursday, Gingrich made clear he wasn't getting Trump's backing.
"No," the former House speaker replied when asked if he was expecting Trump's endorsement. He added that he was amazed at the attention Trump was getting.
The real estate mogul and reality TV show host is known for being unpredictable, and the circus-like atmosphere surrounding the planned endorsement almost seemed designed to gin up interest in the event.
Trump had mused as recently as last month about running for president as an independent and, in interviews, has suggested that he wasn't enthusiastic about Romney's candidacy.
In an interview with CNN last April, Trump dismissed Romney as a "small business guy" and suggested Bain Capital, the venture capital firm where Romney made his millions, had bankrupted companies and destroyed jobs.
"He'd buy companies, he'd close companies, he'd get rid of jobs," Trump said of Romney.
Romney has staked his candidacy on his credentials as a businessman and has pushed back at Gingrich and other rivals who have criticized Bain's practices.
Romney also turned down an invitation to participate in a presidential debate that Trump planned to moderate in Iowa in December. Trump canceled the debate after all the candidates except Gingrich and Santorum refused to participate.
Trump has played an unusually prominent role in the presidential contest since last spring, when he mused publicly about joining the Republican field. His blunt criticism of President Barack Obama and fierce warnings of a nation in decline resonated with tea party activists. At one point, polls showed him briefly surging to the top of the field.
Trump stirred controversy and considerable criticism during that time by openly questioning the validity of Obama's birth certificate, lending credence to the chorus of "birthers" who believe Obama was not born in the United States and thus is ineligible to be president. The fuss pushed Obama to release a long-form version of his birth certificate, proving he was born in Hawaii in 1961.
The president dismissed Trump as a "carnival barker" for ginning up the issue and then memorably skewered his nemesis at the White House Correspondents' Association dinner in Washington, which Trump attended.
Trump announced last May that he would not be a candidate for the GOP nomination. But he welcomed other hopefuls to his office at Trump Tower on Manhattan's Fifth Avenue for strategy sessions. Romney made the trek, as did former candidates Michele Bachmann, Herman Cain and Rick Perry.
Trump even took Sarah Palin out for a widely publicized pizza dinner in Times Square when she was considering a presidential bid. But the 2008 GOP vice presidential nominee ultimately decided against running.
Gingrich visited Trump in December when he was topping polls in Iowa and nationally. After the meeting, Gingrich told reporters he had persuaded Trump to mentor a group of children from some of New York's poorest schools. The gesture came after Gingrich was criticized for suggesting that poor youngsters should do janitorial chores in their schools to learn the importance of work.
Associated Press writer Shannon McCaffrey in Las Vegas contributed to this report.