Barnes made the announcement Wednesday. The Democrat pledged to create jobs and to improve education and transportation. He acknowledged past mistakes, such as feuds with the state's teachers, that some have blamed for his surprising defeat in 2002.
The 61-year-old lawyer is expected to become the early front-runner for the Democratic nomination.
He was ousted from the governor's mansion by Republican Sonny Perdue after serving just one term.
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.