Willem de Ridder (14 October 1939[3] – 29 December 2022) was a Dutch anarchist[4] and artist, known as a founder of Fluxus.

He showed and sold Fluxus works in his gallery, Amstel 47, and shops Fluxshop and European Mail-Order Warehouse.

He organized Dutch Fluxus festivals with Wim T. Schippers in 1963 and 1964.

[3] De Ridder's Amsterdam club, Provadya, was a center for the city's counterculture.

[7] He was part of the Sexual Egalitarian and Libertarian Fraternity (SELF), a group that edited Suck: The First European Sex Paper and organized the first international erotic film festival, the Wet Dream Festival.