Inter, who gained some revenge for their Coppa Italia semi-final defeat by Gennaro Gattuso's side, took the field wearing their kit for next season which features zig-zagged black and blue stripes instead of the traditional straight ones.
Napoli created lots of openings, but it was Inter who struck again in the 74th. Martinez collected the ball in midfield, spun away from his marker, raced towards goal and, as Napoli players backed off, unleashed a 25-metre shot past Alex Meret.
Inter, who have already clinched a top-four finish and a Champions League spot, have 79 points with one match to play, four behind champions Juventus. Conte's team are one point ahead of Atalanta, who beat Parma earlier on Tuesday, with the two set to face off in their final game of the Serie A season on Saturday.
Coppa Italia winners Napoli, who are focusing on their Champions League last-16 second leg match at Barcelona on Aug. 8 with the score at 1-1 from the first game, stayed seventh.