Kerber's 14th career singles title was her first on clay in more than six years.
Juvan upset Kerber at the 2020 French Open and in her first career final put up a remarkable fight. But Kerber prevailed in the longest final of the year.
Kerber, a three-time Grand Slam winner, came to Strasbourg with only two match wins this year and surprised herself with her daily improvement.
She is seeded 21st at the French Open, the only Grand Slam she hasn't won, and starts against Magdalena Frech of Poland on Monday. Juvan will face qualifier Oksana Selekhmeteva, the 2021 girls' doubles champion.