The move gets the Astros' active roster back to 25, with infielder Tyler Greene also joining the club Friday after being acquired in a deal with the Cardinals on Thursday. The Astros optioned infielder Matt Downs and outfielder J.D. Martinez to Triple-A following Thursday's game.
Fernando Martinez was with the Astros earlier this season and hit just .067 in four games before being put on the disabled list after suffering a concussion. He did his rehab in Triple-A and remained there when he was activated from the DL.
The Astros claimed him off waivers from the Mets in January.
Considered one of baseball's top prospects from 2006-10, Martinez, 23, was billed as a power-hitting outfielder who was expected to be a cornerstone of the Mets' lineup throughout this decade. But multiple injuries and an arthritic right knee prevented Martinez from appearing in more than 82 games in any of the past three seasons.
Martinez spent the majority of the 2011 season at Triple-A Buffalo, but he did have a brief stint with the Mets. In 63 games with Buffalo, he hit .260 with eight home runs and 30 RBIs. He also had brief stints with the Mets during the '09 and '10 seasons, appearing in a combined 36 games in that span.