Ciro Immobile cancelled out Mattia Destro's opener at the Stadio Olimpico but the Italy international midfielder popped up with a late goal
The in-form Mattia Destro had opened the scoring at the Stadio Olimpico just before the break only to see the league's top scorer, Ciro Immobile, level matters shortly after half-time with a sublime volley.
However, Florenzi struck in the dying seconds to earn the Giallorossi a win that moves them nine points ahead of Napoli in the battle for automatic qualification for next season's Champions League.
Roma had gone into the game without the suspended Mehdi Benatia, who was replaced in the starting line-up by debutant Rafael Toloi, but they were, at least, buoyed by the return of Daniele De Rossi after a three-game ban.
It was Maicon who was most involved early on, though. The Brazilian was denied what looked like a clear penalty when he was felled in the area by Kamil Glik and then called Daniele Padelli into action with an attempted lob.
The Torino goalkeeper then kept out a well-struck effort from Francesco Totti before pulling off a remarkable double-save to deny Destro and then Miralem Pjanic.
However, Roma finally made the breakthrough four minutes before half-time when Gervinho slipped the ball through for Destro, who prodded the ball past Padelli for his ninth Serie A goal of the season.
The hosts' lead did not last long, though, because Insigne levelled matters in the 52nd-minute with a sensational left-footed volley from an acute angle, after latching onto a long ball from Giuseppe Vives.
Alessio Cerci and Gervinho both squandered decent openings during a frantic finale but an increasingly absorbing encounter looked destined to finish level.
However, in the first minute of injury time, Gervinho produced another astute ball to put Florenzi in on the left-hand side and the Italy international beat Padelli with a sweet low strike to send the Olimpico into raptures.