Cornelis Nicolaas Maria "Cees" Renckens (born 29 May 1946) is a Dutch Doctor of Medicine, gynaecologist, and a well-known skeptical activist against quackery.
From 1988 to 2011, he was president of the Vereniging tegen de Kwakzalverij (VtdK: Dutch Society Against Quackery), which has been actively opposed to all non-science-based medicine since its foundation in 1881.
Renckens has written several books about alternative medicine, pseudoscience and quackery.