ONE Fight Night 21: Eersel vs. Nicolas was a combat sport event produced by ONE Championship that took place on April 6, 2024, at Lumpinee Boxing Stadium in Bangkok, Thailand.

[1] A ONE Lightweight Kickboxing World Championship bout between current champion (also current ONE Lightweight Muay Thai World Champion) Regian Eersel and Alexis Nicolas headlined the event.

[2] A ONE Welterweight Submission Grappling World Championship bout between current champion Tye Ruotolo and Izaak Michell took place at the co-main event.

[3] The bout was originally scheduled at ONE 166 on March 1, but the bout moved at this event for unknown reason.

[4] [6] The following fighters received $50,000 bonuses.