Napoli opened the scoring in the 9th minute as Fabian's curling shot was deflected into the top corner to give Gennaro Gattuso's side an early lead.
But the lead didn't last long, with Immobile connecting with a low cross to steer home from close range for his 36th goal of the season, equalling Gonzalo Higuain's current record.
Lorenzo Insigne restored Napoli's lead on the stroke of halftime, converting from the penalty spot after Dries Mertens was fouled by an errant sliding tackle.
Substitute Matteo Politano sealed the victory for Napoli in stoppage time, tucking the ball into the back of the net after a lovely passing sequence.
The win means Napoli finish the season in 8th place with 62 points, whilst Lazio secure fourth place with 78.
