The Hitech GP driver lined up in fourth but passed Kush Maini for third at the Club chicane on lap four, before swooping past Reece Ushijima for second place at Stowe several laps later.
Championship leader Martins (ART) remained at the front for much of the 17-lap race, having taken the lead off the line from polesitter Ushijima, but Hadjar eventually made it past two laps from the end around the outside of Stowe.
