The 30-year-old took his tally to 36 goals, equalling the record set by Gonzalo Higuain for Napoli in the 2015-16 season. He also finished the season five goals clear of the next leading scorer, Cristiano Ronaldo of Juventus.
Fabian Ruiz gave Napoli a ninth-minute lead with a shot from outside the penalty area which took a deflection and looped into the net, leaving goalkeeper Thomas Strakosha helpless.
Lorenzo Insigne struck the post for Napoli before Immobile equalised in the 22nd minute, sweeping the ball between goalkeeper David Ospina and his near post from Adam Marusic's pass.
Napoli regained the lead from a penalty in the 54th minute, Insigne sending Strakosha the wrong way after Dries Mertens was caught by Marco Parolo when he was about to shoot. Matteo Politano added a third in stoppage time.
Lazio finished fourth with 78 points and Coppa Italia winners Napoli ended the season seventh with 62 points.
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