The 33-year-old's contract with Chelsea expired this summer and was released from Stamford Bridge after the Blues opted not to offer him a contract extension.

Germany's Ballack, who's missing the World Cup due to injury, decided to return to Leverkusen, where he kicked off his career.

According to media reports, Ballack will earn around €12million for his two-year stay at Leverkusen.

"With my transfer to Bayer Leverkusen a circle is complete. I had a beautiful and successful time here, on which I like to look back on a lot," Ballack told the Bundisliga club's official website.

"That was a factor in my decision.

"But above all the good sporting potential of this young and attractive team were persuasive.

"I am especially looking forward to the great fans, who naturally have been key in my decision to return to Bayer."

Ballack spent three years at BayArena before joining Bayern Munich in 2002.

The highly-rated midfielder moved to Chelsea four years later and was a key player with the London side over the past years.