Is Southern France in the Mediterranean Region?

Yes. Southern France (Occitanie, Provence-Alpes-Côte d'Azur, Corsica) borders the Mediterranean Sea. It features a Mediterranean climate (hot dry summers, mild wet winters), vegetation (garrigue, maquis), and iconic cities: Marseille, Nice, Toulon, Montpellier, Perpignan, Ajaccio. It is culturally, climatically, and geographically Mediterranean.