Gang War in Naples was director Pasquale Squitieri's fourth film after directing three spaghetti Westerns. Fabio Testi was cast in the film as Tonino Russo after Squitieri saw him in Le Tueur with Jean Gabin. Squitieri went to Paris to meet him despite his producers wanting to cast Massimo Ranieri and Martin Balsam for the roles. Squitieri explained that he "didn't have anything against those actors, but I needed someone who would scare the audience when he came on the screen, and Fabio Testi, with his imposing physique, was just perfect." Squitieri also cast Raymond Pellegrin, noting that Pellegrin "was my idol, he starred in a movie I adore, André Cayatte's Are We All Murderers?." Squiteri also declared casting Jean Seberg was easy as she was Testi's girlfriend at the time.
