Senator Rand Paul has announced he is launching a 2016 presidential election campaign on the Republican ticket.
"Today begins the journey to take America back," he said in a campaign event in Kentucky.
Mr Paul, a Kentucky senator, stands out from the Republican pack because of his comparatively libertarian views.
He becomes the second Republican to enter the contest that concludes in November 2016.
Speaking in front of a large crowd at the official campaign kick-off, he said: "The Washington machine that gobbles up our freedoms and invades every nook and cranny of our lives must be stopped."
A first-term senator hailing from one of the country's most well-known libertarian families, Mr Paul first held elected office when he rode a wave of Tea Party popularity to the US senate in 2010.
Once there, he soon drew attention when he spoke for more than 12 hours in protest about President Barack Obama's drone policy and the nomination of John Brennan to lead the CIA.
Rand Paul - at a glance
- an ophthalmologist and Kentucky senator
- a libertarian and Republican, also son of Ron Paul, who ran for president several times
- anti-surveillance and anti-drone, but conservative on same-sex marriage and abortion
- attacked by some in his party as being isolationist on foreign policy
- has testy relationship with senior party figures like John McCain and Mitch McConnell
He has proven to be a thorn in the side of many of his fellow Republicans, openly challenging them on issues such as government surveillance, drone policies and sanctions on Iran and Cuba.
He has also questioned the size of the US military and proposed relaxing drug laws that lock up offenders at a high cost for tax payers.
It is not clear how successful Mr Paul will be amongst mainstream Republican supporters.
His father, former member of the US House of Representatives Ron Paul, ran several unsuccessful presidential campaigns that had strong appeal to libertarians who favour limited government and lower taxes.
At the scene: Anthony Zurcher, BBC News
On a stormy Tuesday morning, deep in the bowels of the massive 1,200 room Galt Hotel in downtown Louisville, Kentucky, Rand Paul launched his bid for the 2016 Republican presidential nomination.
Mr Paul doesn't sound like your average Republican presidential candidate, and at times his event didn't look much like a traditional Republican presidential kick-off. Very early on, he criticised both Republicans and Democrats for being creatures of a political establishment that is unresponsive to the American people.
"Too often when Republicans have won we have squandered our victory by becoming part of the Washington machine" he said "That's not who I am."
The morning started with a video by country music singer John Rich featuring a blue-collar factor worker being laid off and the refrain, "Here' in the real world they're shutting Detroit down" - interspersed with wonky sound bites of the Kentucky senator talking about "economic freedom zones".
Mr Paul is expected, however, to run a very tech-savvy campaign that could create appeal to new Republican demographics, like young voters.
Indeed, the campaign's official announcement first came in a post on his website several hours before he appeared on stage in Louisville, Kentucky.
"I am running for president to return our country to the principles of liberty and limited government," he said in the online message.
He joins Senator Ted Cruz as the two most prominent declared candidates.
He could face up to 20 other fellow Republicans doing battle for the nomination, including former Florida Governor Jeb Bush and Wisconsin Governor Scott Walker, both of whom are expected to enter the race soon.
The triumphant individual is widely expected to battle with Hillary Clinton, the Democratic favourite and former US Secretary of State.
Ms Clinton has yet to officially announce her candidacy, but is expected to do so in the next two weeks.
Mr Paul attended Baylor University, but did not graduate. He later attended Duke Medical School.
Mr Paul lives in Bowling Green, Kentucky. His wife Kelly and he have three sons.