Ayotte will make her endorsement official in a campaign stop with Romney in Nashua, according to a message sent to supporters late Saturday night.
"I will be working as hard as I can to help him secure the Republican nomination and, most importantly, ensure that Barack Obama is a one-term president," Ayotte wrote in a message distributed after news of the endorsement became public.
The endorsement represents a major pickup for Romney, who's already leading polls in the early primary state.
The 43-year-old Ayotte will become the first member of the New Hampshire congressional delegation to endorse a presidential contender.
"This is Gov. Romney's biggest endorsement in New Hampshire," said Steve Duprey, a New Hampshire member of the Republican National Committee.
Romney already enjoys the support of several prominent New Hampshire political figures, including former Gov. John H. Sununu and former Sen. Judd Gregg.
But New Hampshire Republicans say the backing of Ayotte, who won a landslide election just a year ago, gives Romney tremendous advantages.
"As the most popular Republican in the state, Kelly represents the future of our party," said Michael Dennehy, a local GOP operative who led Sen. John McCain's presidential campaign four years ago. "Very few endorsements matter in this process, but Sen. Ayotte's adds weight in New Hampshire and nationally."
Ayotte said electability was a factor in deciding which presidential hopeful to support.
"It is imperative that Republicans nominate our strongest candidate to face President Obama," she said.