"I am officially announcing that I am a candidate for president in the Republican primary," Paul said on ABC's "Good Morning America."
Paul, 75, ran a spirited campaign in 2008, capturing the attention of grassroots followers with his talk of smaller government focused more on issues at home, as well as his disdain for the Federal Reserve.
Paul was in New Hampshire -- an early primary state -- for a campaign rally in Exeter Friday.
He posted on his Twitter page Thursday he would make a "major announcement."
In an ABC News-Washington Post poll last month, Paul had the support of only 2 percent of Republicans or Republican-leaning independents, compared to 16 percent for former Massachusetts Gov. Mitt Romney.
Paul, father of Sen. Rand Paul, R-Ky., becomes the second Republican to announce he would seek the nomination. Earlier this week, former House Speaker Newt Gingrich of Georgia announced he would run for the GOP nomination.