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.
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'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.
