The 26-year-old right-hander held Pittsburgh to one run on seven hits over seven innings, tying a career-high with nine strikeouts.
Arrieta, who was demoted to the bullpen following his last start Friday, has lost eight of his last nine decisions heading into the day, but was given the start Wednesday after left-hander Brian Matusz bunted a ball off his nose Tuesday during pitchers batting practice.
That puts Arrieta in line for his first win since May 2, when he threw eight shutout innings at Yankee Stadium.
Against a starting lineup that had just two batters hitting over .248, Arrieta held the Pirates scoreless for his first six innings of work. Pittsburgh scored their only run off him with a two-out RBI ground-out in the seventh to make the score 5-1.
The Orioles scored two or fewer runs in nine of Arrieta's 13 starts this season, but the the Orioles currently lead 7-1 in the top of the ninth. Chris Davis and Wilson Betemit have both homered and have three RBI apiece.