"Today I have withdrawn my candidacy for president of the United States," Thompson said in a statement posted on his campaign Web site. "I hope that my country and my party have benefited from our having made this effort."
His decision to abandon his campaign came after poor finishes in the Nevada caucus and the South Carolina Republican primary.
The former senator from Tennessee, also known as District Attorney Arthur Branch on NBC's "Law & Order," entered the race late and said he needed a strong showing in South Carolina to continue. Thompson came in third behind Sen. John McCain, R-Ariz., and former Arkansas Gov. Mike Huckabee in Saturday's primary.