Romney, 65, won the nomination as he crossed the support of necessary 1,144 delegates during the ongoing Republican National Convention here.
The convention also approved Congressman Paul Ryan as his vice presidential running mate. Romney's nomination will become official after he delivers his acceptance speech tomorrow, campaign officials said.
He will legally be able to spend money raised for the general-election campaign only after Thursday's speech.
His wife Anne Romney took the center stage in Tampa to tell her fellow Americans that the country needs him and urged them to give him a chance to lift America.
The elevation of Romney at the Republican National Convention completed his six-year quest to navigate the contours of a changing party.
Romney, former Massachusetts governor, swept into the convention from backstage after watching his wife deliver a deeply personal speech that made a direct appeal to female voters, whom she assured: "You can trust Mitt."
In a well-choreographed effort led by his wife, Anne said, "This is the man America needs."
"This is the man who will wake up every day with the determination to solve the problems that others say can't be solved, to fix what others say is beyond repair. This is the man who will work harder than anyone so that we can work a little less hard," she said.
"This is our country. This is our future. These are our children and grandchildren. You can trust Mitt. He loves America. He will take us to a better place, just as he took me home safely from that dance. Give him that chance," Anne said.
Romney arrived for the convention, earlier than scheduled, with his campaign keeping a worried eye on Hurricane Isaac as it made landfall in Louisiana and threatened the Gulf Coast.