Napoli are seventh in the table with 48 points, 10 points behind fourth-placed Bologna, who played out a goalless draw at Frosinone earlier on Sunday. Monza are 11th with 42 points.
Milan Duric nodded in a goal for Monza nine minutes into the match, outmanoeuvring Napoli's Juan Jesus to connect with a cross.
Napoli's response kicked off 10 minutes into the second half with Victor Osimhen's equaliser, closely followed by Matteo Politano's remarkable volley two minutes later. In the 61st minute, Piotr Zielinski's long-range strike secured the visitors' third goal.
Andrea Colpani swiftly added to Monza's tally with a stunning curled shot into the corner a minute later, while Giacomo Raspadori capitalised on a rebound to score Napoli's fourth goal, 68 minutes into the game.