McDyess, 34, will sign a three-year deal that guarantees him $15 million.
McDyess, a 6-10 forward, averaged 9.6 points and 9.8 rebounds for the Pistons last season.
The Spurs have a successful summer and thrust themselves back into championship contention. General manager R.C. Buford has retooled the team with Richard Jefferson, McDyess and a potential second-round steal in Pittsburgh forward DeJuan Blair.