Romney won the nomination as he crossed the support of necessary 1,144 delegates during the ongoing Republican National Convention in Tampa, Florida on Tuesday.
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 on Thursday, campaign officials said. He will legally be able to spend money raised for the general-election campaign only after Thursday's speech.
Romney, 65, is the first Mormon to be nominated as the Republican presidential candidate.
Mormons are a religious and cultural group related to Mormonism, the principal branch of the Latter Day Saint movement which began with the visions of Joseph Smith in upstate New York during the 1820s.
65-year-old Romney will now contest against Barack Obama in the November polls to occupy the White House as new US president. Latest opinion polls indicate that Obama and Romney are in a statistical tie with each other.
Meanwhile during the convention, Republican speakers lashed out at President Obama and condemned the policies followed by him during his term as the president.