Riefenhauser signed a minor league deal with the Astros on Tuesday, which includes an invite to spring training, the Houston Chronicle's Jake Kaplan reports.
Riefenhauser had a 4.55 ERA and 1.27 WHIP in 27.2 innings over 28 games with Triple-A Iowa before he was released in August. The 26-year-old southpaw will look to earn a spot in the Astros' bullpen, which had the 10th-best ERA in the league last season.