Simone Inzaghi's Lazio went into the match looking to bounce back from their 3-1 derby defeat by Roma last weekend, and did just that to draw level on points with second-placed Napoli, who face Sassuolo later in the day.
"Today we saw the real Lazio, the team with heart that battles hard, that wins duels and games," midfielder Senad Lulic told DAZN.
"If we play like this, then we can beat anyone. You could see today how hungry we were for the three points."
Marco Parolo flashed a shot narrowly wide early on, before Lazio defender Wallace got away with a slack pass as Marco Benassi's effort was kept out by Thomas Strakosha.
Immobile made the breakthrough in the 38th minute, as Stefan Radu nodded on a corner and the Italy striker stretched to volley in at the back post for his fifth league goal of the season.
The 28-year-old, who signed a contract extension which will keep him at the club until 2023 earlier this week, has now scored 57 Serie A goals since the start of the 2016-17 campaign.
Giovanni Simeone had a penalty appeal waved away for the visitors, while Marko Pjaca also went close to an equaliser, but the home side held on for their fourth win in five league games.