Andreeva, 19, did not fall the way of so many of the leading contenders at a shock-filled tournament as she dominated to win 6-1 6-3 in just 76 minutes.
Kostyuk, competing in her first major semi-final having won 17 matches in a row on clay this season, was under pressure in every service game and only managed to hold twice against an impressive opponent.
“I was very nervous coming into this match,” said Andreeva.
“She has had an amazing season and until this match she hadn’t lost on clay, so of course that puts a lot of pressure. She is an amazing player and tough opponent and I’m just super happy with the way I played today.
“I am also happy that I got revenge from the Madrid final [where she lost to Kostyuk]. I am also happy to reach my first Grand Slam final, all of these feelings combined it’s amazing, I’ve never felt anything like this before and I am just very excited about the last match here in Paris.”
Andreeva will face either 25th seed Diana Shnaider, her regular doubles partner who upset world No. 1 Aryna Sabalenka in the quarter-finals, or qualifier Maja Chwalinska in the women’s final on Saturday.
“I told myself to accept everything that happens today on the court because today was one of those days where I felt like everything could happen and it was a bit unpredictable,” added Andreeva.
“I told myself that no matter what happens I am going to fight, give my best and if she ends up winning she will have to really work for it and fight. With this kind of mindset I ended up winning the match and I am very happy about it.”
After all the upsets seen so far at the French Open, this was an impressively composed performance from Andreeva, just a month after losing to Kostyuk in straight sets in the Madrid Open final.
The 19-year-old took advantage of a nervy start from Kostyuk with a break in the first game.
Andreeva got a double break as she finished off a rally with a smash and was soon into a 4-0 lead.
The pressure continued on Kostyuk’s serve as she managed to fend off five break points to win the fifth game.
But Andreeva followed up with a love hold and then wrapped up a dominant set.
There was a chance for Kostyuk to get the upper hand early in the second set but she netted a mid-court forehand on break point.
Andreeva responded by reeling off three of the next four games to take control of the set.
A surprise mid-game closing of the roof on Philippe Chatrier looked like it might throw Andreeva off as she was broken to love.
But she quickly recovered by breaking straight back and then holding to close out the win.