Reinier de Ridder (born September 7, 1990), also known by his initials RDR, is a Dutch professional mixed martial artist. He currently competes in the Middleweight division of the Ultimate Fighting Championship (UFC). He previously competed in ONE Championship, where he is a former ONE Middleweight World Champion and ONE Light Heavyweight World Champion. De Ridder was the third two-division champion in ONE history, having previously held the middleweight and light heavyweight belts simultaneously. As of March 11, 2025, he is #13 in the UFC middleweight rankings.