Joaquin Correa finished off a superb team move to put the Biancocelesti in front in the first half, before Immobile calmly slotted home late on.
It was an important strike, sending the Italy international one goal clear of Bayern Munich’s Robert Lewandowski at the top of the European golden boot standings and within one strike of equalling the Serie A single season scoring record of 36, set by Gonzalo Higuain at Napoli in 2015/16.
Lazio remain in fourth place, level on 78 points with Atalanta and one point behind Inter, but a win at Napoli on the final day of the season on Saturday would guarantee them a top-three finish, as La Dea face the Nerazzzurri.
Lazio took the lead after 17 minutes when Correa finished off a flowing team move by exchanging passes with Immobile before firing home his ninth league goal of the season.
The visitors had goalkeeper Lorenzo Andrenacci to thank for keeping the score down when his one-handed save denied a fierce Luis Alberto drive, before he leapt to push away a glancing Immobile header.
Immobile kept pushing for his goal after the break, rustling the side netting with one effort before shooting just wide when one-on-one with the keeper late on.
However, his moment came after 82 minutes when Correa’s pass sent him clean through on goal and this time he made no mistake, firing into the bottom corner.
Lazio almost stretched their lead further late on when Alberto fired a close-range shot off the crossbar, before Andrenacci performed two more superb reaction saves to deny Immobile a brace in stoppage time.