The Sweden striker scored 15 minutes from time to send PSG into the Stade de France showdown on May 21 against Ligue 2 side Sochaux or bitter rivals Olympique de Marseille, who play on Wednesday.
PSG, who were knocked out of the Champions League quarter-finals for the fourth season in a row last week, are gunning for a second consecutive domestic treble.
They were crowned Ligue 1 champions last month and will be playing the League Cup final against Lille on Saturday.
PSG, however, were far from impressive at Lorient and managed a single shot on target in the first half.
They were slightly better after the break and Ibrahimovic scored his 42nd goal of the season in all competitions when he slotted the ball home from inside the box after being played through by Angel Di Maria.
Zargo Toure came close to levelling one minute from time but his header was superbly saved by Salvatore Sirigu.
(Reporting by Julien Pretot, edited by Alan Baldwin)
