Reinier de Ridder[3] (born September 7, 1990), also known by his initials RDR, is a Dutch professional mixed martial artist.

De Ridder was the third two-division champion in ONE history, having previously held the middleweight and light heavyweight belts simultaneously.

After this winning streak, Reinier got the opportunity to fight for his the Middleweight belt in HIT FC, which he won by first round rear-naked choke.

[4] De Ridder made his ONE debut at ONE: Hero's Ascent on January 25, 2019 against Fan Rong, in a fight which he won by a first round submission.

De Ridder faced Aung La Nsang for the ONE Middleweight World Championship at ONE: Inside the Matrix on October 30, 2020.

[13] After Vitaly Bigdash tested positive for COVID-19, De Ridder stepped in for a rematch against Aung La Nsang for the ONE Light Heavyweight World Championship at ONE on TNT 4 on April 28, 2021.

[19] De Ridder faced former multiple-time ADCC and BJJ world champion André Galvão in a grappling match at ONE: X on March 26, 2022.

[27] De Ridder faced Anatoly Malykhin to defend his light heavyweight title on December 3, 2022, at ONE on Prime Video 5.