Nikki Haley is dropping out of the 2024 presidential race after winning just two Republican primaries.
"The time has now come to suspend my campaign," she said at a news conference on Wednesday morning.
Her departure will leave her rival Donald Trump as the only Republican left after he dominated Super Tuesday, racking up a series of wins.
Ms Haley did not endorse Mr Trump during her exit speech.
Instead, she congratulated him and said she "wish[ed] him well
She said it is now up to Mr Trump "to earn the votes of those in our party and beyond it who did not support him".
Though Ms Haley came nowhere near to securing the party's nomination, she found substantial support among voters who had rejected the former president, and in particular college-educated voters.
The question now will be whether her supporters, who made up a quarter of Republican primary voters in some states, will back Mr Trump.
On his social media site Truth Social, Mr Trump said Ms Haley had been "trounced last night, in record setting fashion".
Ms Haley had pitched herself as a fresh start for the party and the chance to move past Mr Trump. But she only won Vermont and the District of Columbia in the primaries.
Ms Haley has also not yet spoken about her plans for after the primary race.
During her speech on Wednesday, Ms Haley said she was looking forward to life as a "private citizen".
"Although I am no longer a candidate, I will not stop using my voice for the things I believe in," she said.
Ms Haley had presented herself as a chance for the country to move on from the "chaos" of her former boss, who made her ambassador to the UN.
In the final weeks of her campaign she levelled increasingly forceful attacks at Mr Trump, taking aim at his age and mental acuity, and criticising him for "buddying up" to dictators like Russia's Vladimir Putin and North Korea's Kim Jong-un.
The former president even beat her in South Carolina where she was a popular governor.
Ms Haley said on Wednesday she stayed in the race because she had "wanted Americans to have their voices heard" and "I have done that".
"I have no regrets," she said.
During her brief speech, the former United Nations ambassador emphasised the policy issues of her campaign stops, including the economy, the threat of socialism, and international conflict.
"Our national debt will eventually crush our economy," she said. "Our world is on fire because of America's retreat. Standing by our allies in Ukraine, Israel and Taiwan is a moral imperative."
On Super Tuesday Mr Trump swept Ms Haley by double-digit margins in several states, including Texas and California, the two states offering the largest number of delegates.
Her departure puts Mr Trump on a clear path to win the Republican nomination this month unchallenged and a rematch with Joe Biden in November.
Republican Senate Minority Leader Mitch McConnell - who had yet to endorse a candidate - quickly announced his support for Mr Trump after Ms Haley's announcement. The two have sparred in the past, but Mr McConnell said it was "abundantly clear" Mr Trump had earned the support of the party.
Some criticised Ms Haley for declining to endorse Mr Trump, including Penny Nance, the leader of the conservative Concerned Women for America group. She argued Mr Trump was the best chance to defeat Democrats.
"If [Haley] is not on board with that, her motives are clear and conservatives will never forgive or forget," she said.
Republican National Committee chair Ronna McDaniel, meanwhile, commended Ms Haley for a "hard-fought campaign".
Mr Biden was quick to try to woo the former South Carolina governor's supporters after her announcement.
"It takes a lot of courage to run for President - that's especially true in today's Republican Party, where so few dare to speak the truth about Donald Trump," he said of Ms Haley. "Donald Trump made it clear he doesn't want Nikki Haley's supporters. I want to be clear: There is a place for them in my campaign."
Mitch Landrieu, a national co-chair of the president's re-election campaign, told CNN that Mr Biden had personally contacted Ms Haley on Wednesday.
"There are a lot of the things that the folks that voted for Nikki Haley share with Joe Biden," he said.
In a statement on Tuesday night, prior to Ms Haley ending her run, her campaign said it was "honoured" she had become the first Republican woman to win two presidential primary contests.
"There remains a large bloc of Republican primary voters who are expressing deep concerns about Donald Trump.
"That is not the unity our party needs for success. Addressing those voters' concerns will make the Republican Party and America better."
