Ruffier blocked Thiago Silva's weak spot-kick to effectively seal the home team's passage into the last four after a tense affair at the Geoffroy Guichard ended 0-0 after extra time.
"It was not easy. The whole match was tough against a great PSG team," the keeper told French TV channel France 3.
Ruffier started brightly and never lost concentration in a game dominated by PSG whose forwards just could not beat him.
PSG's Zlatan Ibrahimovic intercepted an awful pass from a Saint-Etienne defender and rushed into the box but Ruffier kept his composure to parry the Swede's low shot with his right foot.
Saint Etienne responded quickly but Pierre-Emerick Aubameyang's header was cleared off the goal-line by Clement Chantome.
PSG striker Ezequiel Lavezzi then had a goal disallowed for offside and while Les Verts had most possession, the visitors looked more dangerous.
PSG's second-half substitute Nene provided more pace up front but they still struggled to create any clear chances in a match featuring the best two defences in Ligue 1.
In extra time, Lavezzi collected a terrible backpass by Francois Clerc but his own poor delivery to Ibrahimovic enabled Ruffier to dive at the striker's feet to deny PSG once again.
Lavezzi fell on his knees in disbelief while former AC Milan striker Ibrahimovic hit the ground in anger.
PSG full-back Sylvain Armand unleashed a spectacular rising shot from the edge of the box in the closing stages but Ruffier tipped the ball over the bar to send the tie into a shootout.
On Wednesday, French champions Montpellier host Nice while Lille travel to Bastia. Stade Rennes host Troyes on Thursday.
The semi-final draw will also be made on Thursday.
