Florida Senator Marco Rubio has told donors that he will run for the Republican nomination for US president in 2016, US media report.
Mr Rubio, 43, said on a conference call he was “uniquely qualified” to bring the party into the future.
He is the third Republican to officially announce a candidacy after Senators Rand Paul and Ted Cruz.
Mr Rubio is expected to make a formal announcement at a political rally in Miami later on Monday.
It comes a day after former Secretary of State Hillary Clinton said she would stand for the Democratic nomination.
Mr Rubio, a Cuban-American, has been a harsh critic of President Barack Obama’s policies, especially on immigration and the diplomatic thaw with Cuba.
“The Republican Party, for the first time in a long time, has a chance in this election to be the party of the future,” Mr Rubio said on the call, according to the Associated Press.
“Just yesterday, we heard from a leader from yesterday who wants to take us back to yesterday, but I feel that this country has always been about tomorrow,” Mr Rubio added, referring to Mrs Clinton, a former first lady and US senator.
The Florida senator, who was first elected in 2010, holds conservative positions on government and military spending, abortion and negotiations with Iran over its nuclear programme.
He was previously criticised by some Republicans for initially supporting a bipartisan Senate immigration reform bill.
He has since said that border security must be strengthened before any change, criticising Mr Obama’s executive actions on immigration.
He is due to speak to supporters at the Freedom Tower in Miami at 18:00 local time (22:00 GMT).
Analysis, Anthony Zurcher, North America reporter
Reporters are already gathering in the subtropical heat outside downtown Miami’s Freedom Tower, the newspaper-building-turned-art-museum that served as a government processing facility for Cuban refugees in the 1960s.
The symbolism of this site for a presidential announcement for Florida Senator Marco Rubio is obvious.
The son of working-class Cuban immigrants, Mr Rubio will make his “American Dream” success story – and Latino ancestry – a driving narrative of his campaign.
Mr Rubio will likely try to paint a sharp contrast with Mrs Clinton who could be his opponent next year – and convince Republicans he’s their best hope for victory.
After all, it was another fresh-faced senator with a gift for rhetoric and a compelling personal narrative who bested Mrs Clinton when she ran for president in 2008.
Although Mr Rubio may shy from the comparison, the parallels are clear. Is Marco Rubio the next Barack Obama?
Mr Rubio would be the first Hispanic president if he won, as would Texas Senator Ted Cruz.
The field for the Republican nomination is likely to be crowded, with former Florida Governor Jeb Bush and Wisconsin Governor Scott Walker also expected to run.
Mrs Clinton, the first major candidate on the Democratic side to declare, is travelling to Iowa and other states, seeking to meet voters before a more formal rally in May.
2016 runners and riders
- An early (undeclared) Republican frontrunner is Jeb Bush
- Hillary Clinton will have learnt much from her failed campaign of 2008
- New Jersey Governor Chris Christie could battle Bush for the party’s centre ground
- Texas Senator Ted Cruz is a darling of the Tea Party
- Libertarian Rand Paul has his supporters – and enemies – among Republicans
- Firebrand liberal Elizabeth Warren is championed by many in the Democratic Party, but insists she’s not running.
Meet the 2016 hopefuls