"It's been a long time coming, and I'm happy to be back here," Major League Baseball's website (www.mlb.com) quoted him as saying at a Chicago news conference.
"It took me a while to get to this point, but I had to get baseball out of my system.
"I'm at peace with it ... I had one heck of a career."
Thomas, known as 'The Big Hurt', played for 16 seasons with the White Sox before finishing his career with Oakland and Toronto. He had not played since 2008.
The 41-year-old is one of four players to have a .300 average, with 500 home runs, 1,500 RBIs, 1,000 runs scored and 1,500 walks in his career. The others are Hall of Famers Mel Ott, Babe Ruth and Ted Williams.
"Everyone who enjoyed watching Frank Thomas perform during his outstanding career with the White Sox quickly realized we were watching one of the greatest offensive players of all-time," White Sox chairman Jerry Reinsdorf said in a statement
Thomas won back-to-back American League Most Valuable Player honors in 1993 and 1994.
His number 35 will be retired during 'Frank Thomas Day' on August 29 prior to the White Sox's game with the New York Yankees in Chicago, the team said.
(Reporting by Gene Cherry in Salvo, North Carolina. Editing by Ian Ransom)