The multi-year deal will mean Agassi will wear footwear and clothing by Adidas for the first time since signing with Nike in 1988.
The 35-year-old US player announced the sponsorship switch at the Mercedes-Benz Cup, where he hopes to play his first match since an opening-round loss at the French Open.
A back injury has kept Agassi off court since May, the same injury that stopped him playing Wimbledon for the second year in a row this year.
The new sponsorship deal means Agassi and his German tennis player Steffi Graf will be signed to Adidas. "I'm excited about my new partnership," Agassi said. Agassi and Graf previously starred together in an ad for T-Mobile in 2003.
The deal will see Agassi join the British number one Tim Henman, who is also sponsored by Adidas.
It is believed Agassi choose to endorse Adidas because the company has promised to support the tennis ace's charity endeavours, including his Las Vegas-based children's foundation.
Nike first signed Agassi in 1988 at the height of his tennis career.
He soon became as famous for winning major tournaments, including eight Grand Slams and Wimbledon in 1992, as he was for his bright tennis clobber and his long-since gone ponytail.
Nike spends millions of pounds on sponsorship of sports stars including tennis star Serena Williams, golf player Tiger Woods and across the Brazilian football team.
Adidas' endorsers include England football captain David Beckham and rugby ace Jonny Wilkinson.
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