Lazio captain Immobile struck low into the left of the net in stoppage time, sending Fiorentina goalkeeper Pietro Terracciano the wrong way after Lazio were awarded a penalty when Fiorentina's Nikola Milenkovic handled the ball.
Fiorentina are sixth in the standings on 17 points, one point behind champions Napoli in fifth and two less than fourth-placed Atalanta, who beat lowly Empoli 3-0 earlier on Monday.
Lazio, who won three consecutive home games in the competition for the first time since April, moved up to seventh in the standings on 16 points.
Lazio next travel to Bologna on Friday while Fiorentina take on second-placed Juventus on Sunday.
Reporting by Anita Kobylinska in Gdansk Editing by Christian RadnedgeOur Standards: The Thomson Reuters Trust Principles.
Persons:
Ciro, Remo Casilli, Vincenzo Italiano's, Pietro Terracciano, Fiorentina's Nikola Milenkovic, Atalanta, Maurizio Sarri, Lucas Beltran, Felipe Anderson, Anita Kobylinska, Christian Radnedge
Organizations:
Soccer, Lazio, Fiorentina, Stadio Olimpico, Rights, Serie, Napoli, Empoli, Inter Milan, Sky Sports, Serbian, Europa Conference League, Juventus, Sunday, Thomson
Locations:
Rome, Italy, Bologna, Gdansk