Pedro Feliciano earned his second save, retiring the final two hitters with two runners on after Pittsburgh scored three times in the ninth against rusty rookie Eddie Kunz, who hadn't pitched since Sunday.
Before the game, New York got disappointing news regarding its shaky bullpen when injured closer Billy Wagner felt discomfort in his pitching elbow following a simulated game.
The All-Star reliever, sidelined with a strained left forearm, won't come off the disabled list as expected Monday and instead will return to New York on Sunday for further evaluation.
Martinez (4-3) allowed one run, eight hits and struck out four in his 13th start of the season. His longest outing prior to Saturday was 6 1-3 innings in a loss to San Diego on Aug. 6.
The oft-injured right-hander, who won for the first time since beating the Phillies on July 7, has gone seven innings just twice in his past 22 starts dating to 2006.
Reyes led off the game with a home run against Zach Duke (4-11) and added an RBI triple in the second.
Carlos Beltran and Fernando Tatis had RBIs as part of a three-run first, and Castro's two-run homer in the sixth made it 6-0. Pinch-hitter Daniel Murphy drew a bases-loaded walk in the eighth.
Duke, who retired 13 of 14 batters during one stretch, allowed six runs and nine hits in six innings. The left-hander, winless in 12 starts, dropped to 0-5 with a 7.97 ERA in the second half.
Jack Wilson and Freddy Sanchez had three hits apiece, and Andy LaRoche homered for the Pirates, who have lost three straight.
Each starter had at least one hit for the Pirates, who had a runner on base in every inning.
Notes: Pirates general manager Neal Huntington credited team owner Bob Nutting with providing the financial backing to sign No. 2 overall pick Pedro Alvarez. The Vanderbilt slugger received a franchise-record $6 million signing bonus. "We were given the resources to do what we needed to," Huntington said Saturday. "Because of that, we sit here today a much deeper and stronger organization." ... Reyes' leadoff homer was the 12th of his career, extending his team record. His triple was his major league-leading 14th and the 66th of his career, also a club record. ... The Mets have scored 96 first-inning runs. ... Pittsburgh drew 37,506, its second consecutive sellout.
