LeBlanc plans to join the team in time for Sunday's series finale in Kansas City, at which point the Astros will make a corresponding roster move.
LeBlanc went 1-5 with a 5.18 ERA in seven starts for the Marlins this season. He's 20-32 with a 4.47 ERA in six major league seasons spent with San Diego and Miami.
Astros manager Bo Porter said he's familiar with former second-round pick going back to LeBlanc's days with the Padres. His best season was in 2010, when he was 8-12 with a 4.25 ERA.