The Pirates pulled Nady after the first inning Friday night against San Diego.
Nady was in right field during the top of the first but was replaced by Jason Michaels in the field in the second.
Marte, one of the most coveted left-handed specialists in the game, was caught by television cameras hugging teammates in the dugout after leaving the bullpen shortly after.
"There is a trade in place, pending medical reviews of all the players involved on both clubs," Pirates director of media relations Jim Trdinich said. "That is all that is permissible to be announced at this time."
The Tampa Bay Rays, New York Yankees and New York Mets all were thought to be interested in Nady, who was batting .330 with 13 home runs and 57 RBIs. Marte is 4-0 with a 3.47 ERA and 47 strikeouts in 46 2/3 innings, and also was attracting interest from several teams.
