Ukrainian Kostyuk is ranked 15th in the world, but was strongest on the court when she faced world number three Swiatek in Paris.
Kostyuk won in two straight sets (7–5, 6–1) in the fourth-round match.
"I'm still in shock. To beat such an incredible player who has won here four times, it's incredible, I can't believe it," Kostyuk said in the post-match interview, thanking the crowd for the energy boost.
Iga Swiatek has won the competition four times, most recently in 2024.
On Saturday evening, the reigning French Open champion, Coco Gauff, also went out after losing to 28th-ranked Anastasia Potapova.
