Sources told ESPN.com that Baynes has agreed to a three-year deal with the Pistons that will include a player option that allows him to return to free agency after Year 2.
Although the financial specifics weren't immediately known, one source said that the value of the deal could rise as high as $20 million.
Baynes is regarded as an on-the-rise big man whom San Antonio wanted to keep. But the Spurs had little hope of retaining Baynes given their all-out pursuit in free agency of Portland Trail Blazers All-Star forward LaMarcus Aldridge.
The 28-year-old thus becomes the second power player to leave San Antonio this week before the Spurs have secured a commitment from Aldridge. On Wednesday, San Antonio and Atlanta verbally agreed to a trade that will send Tiago Splitter to the Hawks on July 9, which is the first day trades and signings such as Baynes' deal can be completed after the lifting of a leaguewide moratorium on roster business.