Sergio Floccari's injury-time goal sent Lazio into the Coppa Italia final at the expense of Juventus, who looked to have sent the tie into extra-time when Arturo Vidal equalised moments earlier.
Floccari rose highest to head home from a corner deep into injury-time to give his side a 2-1 win on the night, and a 3-2 aggregate triumph.
His goal came mere moments after Vidal looked to have sent the tie into extra-time after stepping inside his marker to rifle into the net from close range.
Lazio midfielder Alvaro Gonzalez broke the deadlock in the 53rd minute with a fine diving header and it looked like that would be enough to separate the teams, who drew 1-1 at in Turin last week, before the late drama occurred.
The Biancocelesti will meet either fierce rivals Roma or Inter Milan at the Stadio Olimpico in the final. Roma must wait until April 17 to try and consolidate their 2-1 first-leg lead.