Delegates from all US states and territories convened to pledge their votes to the former governor of Massachusetts.
He will challenge Barack Obama in November's presidential election.
Recent opinion polls show Mr Romney and the Democratic president locked in a tight race.
Mr Romney, 65, needed to attract at least 1,144 delegates' votes to secure his party's nomination, and the votes from the New Jersey delegation put him "over the top".
Minutes later, Wisconsin Congressman Paul Ryan was formally selected as the Republican vice-presidential candidate.
Ann Romney's moment Mr Romney was chosen in a lively voice poll during which state delegates called out their team's allocation of votes.
Altogether, Mr Romney secured 2,061 votes, House of Representatives Speaker John Boehner announced to the floor of the convention.

He had amassed enough delegates to secure the nomination in May, after a gruelling season of state-by-state primary elections.
But his selection in Tampa put the party's formal stamp of approval on his campaign.
Convention speakers attacked Mr Obama through the session.
Republican National Committee chairman Reince Priebus said the president has "never run a company. He hasn't even run a garage sale or seen the inside of a lemonade stand."
Meanwhile, Mr Boehner said: "His record is as shallow as his rhetoric."
On Tuesday evening, Ann Romney as well as South Carolina Governor Nikki Haley and New Jersey Governor Chris Christie are due to speak.
Mrs Romney's speech is expected to shed light on what her husband is like in private since Mr Romney, formerly a business star and governor of Massachusetts, is seen as something of an unknown quantity by many voters.
The would-be first lady is due to talk about her marriage and the way her husband helped her deal with multiple sclerosis and breast cancer.
In excerpts from her prepared remarks, Mrs Romney, 63, says: "This is the man America needs. This man will not let us down," the AFP news agency reported.
Mrs Romney will also dispel any notion that she has had a "storybook marriage".
"Well, in the storybooks I read, there were never long, long, rainy winter afternoons in a house with five boys screaming at once," she is expected to say.
Vice-presidential candidate Ryan is scheduled to speak on Wednesday evening, while Mr Romney's speech is due on Thursday.
Abortion and Wall Street The convention also approved its party platform - a policy agenda that calls for tax cuts to revive the economy, repealing and replacing a healthcare law passed by Mr Obama, and an end to abortion.
Recent opinion polls have indicated that voters view the economy and unemployment, which is stuck at 8.3%, as top priorities.

The platform also calls for the overturning of measures passed to regulate Wall Street in the wake of the 2008 economic collapse.
On abortion, the platform says: "The unborn child has a fundamental individual right to life which cannot be infringed."
That is more conservative than Mr Romney's declared view - he says he opposes abortions, except in cases of rape, incest, or when the mother's health is in danger.
This is Mr Romney's second run for the White House, after an unsuccessful bid in 2008.
President Obama's renomination will be confirmed next week at the Democratic National Convention in Charlotte, North Carolina.
This year's convention got off to a late start when Monday's programme was postponed amid concerns that Hurricane Isaac might disrupt the proceedings in Tampa.
But the category one hurricane missed Tampa, instead making landfall in southern Louisiana on Tuesday evening.
It comes almost seven years to the day since Hurricane Katrina devastated New Orleans.