Goals from Alexandre Lacazette and Bafetimbi Gomis saw Les Gones build a two-goal lead against Troyes, but the Ligue 2 side made a contest out of it with Thiago Xavier scoring the final goal to ensure the game finished 2-1.
Lyon, who were beaten by Marseille in the 2012 final, will return to Paris on Saturday, April 19 with head coach Remi Garde knowing full well his team are the underdogs.
Remi Garde: "We'll be Tom Thumb.
"But at the end of the day we've qualified for the final, so there's a lot of joy and satisfaction."
PSG have beaten Lyon four times in a row in all competitions and were impressive when seeing off Nantes 2-1 in their own semi-final, which saw Zlatan Ibrahimovic hit a stunning left-footed lob and score a late headed winner.
Midfielder Maxime Gonalons: "Paris are a huge machine and are maintaining an infernal pace in the league.
"You could see how, against Nantes on Tuesday, they have exceptional players like Zlatan who score marvellous goals.
"It will be difficult but, in a match, anything is possible. There's nothing to lose."