The Boston Herald reported the Red Sox were forced to release Smoltz, a long-time star with the Atlanta Braves, after failing to make a trade for him.
Smoltz was designated for assignment Aug. 7, one day after he gave up eight runs, nine hits and four walks in 3 1/3 innings against the New York Yankees. The loss dropped him to 2-5 on the season, and raised his earned run average to 8.33.
Smoltz had signed a one-year deal with Boston in January after more than two decades with the Braves.
Off-season shoulder surgery and subsequent rehab delayed his Red Sox debut until June 25. Only twice in eight starts did he allow fewer than five earned runs, and only twice did he pitch six innings.
Once Smoltz clears unconditional waivers Wednesday, he is free to negotiate with any other team.