Bench coach Dave Trembley was also fired, the team announced.
Porter was in his second season as the Astros' manager. In 2013, Houston was 51-111 and is currently 59-79 in 2014.
Tom Lawless was named the team's interim manager for the remaining 24 games. Lawless managed three different Houston minor league affiliates from 2009 to 2011. He also spent seven seasons managing teams in the Orioles, Padres, Cardinals and Angels systems from 1994 to 2004.
Former Astros infielder Adam Everett, who had been serving as a minor league instructor for the franchise, will be the interim bench coach.
GALLERY: WHEN BASEBALL MANAGERS WERE PLAYERS
- Dan Gartland