Gravel, a former Democratic senator from Alaska, said in an e-mail that the Democratic Party "no longer represents my vision for our great country."
"It is a party that continues to sustain war, the military-industrial complex and imperialism - all of which I find anathema to my views," he said in the e-mail in which he also asked supporters for campaign donations.
Gravel, 77, has been excluded from recent Democratic debates because he failed to meet fundraising or polling thresholds.
"I look forward to advancing my presidential candidacy within the Libertarian Party, which is considerably closer to my values, my foreign policy views and my domestic views," he said.
Gravel left the Senate in 1981 after losing the 1980 Democratic primary. An outspoken critic of the Vietnam War, he was among the first Democrats to speak out against the war in Iraq while others supported the president.
He announced his bid for the presidency on April 17, 2006.