Barnes, who previously served just one term, made the announcement surrounded by family at a news conference Wednesday in Marietta. The Democrat says he wants to improve education and transportation and says he's learned from past mistakes.
Several political insiders expect Barnes will become the front-runner for the Democratic nomination.
Barnes was ousted from the governor's mansion in 2002 by Sonny Perdue, then a little-known state senator. In recent months, the 61-year-old lawyer has been publicly flirting with a comeback.
Three Democrats already in the race are Attorney General Thurbert Baker, House Minority Leader DuBose Porter and former Georgia National Guard Commander David Poythress. Six Republicans are also running.