Nineteen-year-old Marketa Vondrousova is the first teenage finalist at the French Open in more than a decade.
The 38th-ranked Czech player beat 26th-seeded Johanna Konta 7-5, 7-6 (2) to reach the final without dropping a set.
The last teenager to reach the final at Roland Garros was Ana Ivanovic, who was also 19 when she finished runner-up to Justine Henin in 2007.
Vondrousova saved three set points while trailing 3-5 in the opening set.
Vondrousova will face either eighth-seeded Ash Barty, who will also be in her first Grand Slam final on Saturday.
The eighth-seeded Barty ended the run of 17-year-old American Amanda Anisimova 6-7 (4), 6-3, 6-3 on Friday.