The malacologist Wim Maassen was an autodidact and citizen scientist, with a primary interest in land- and freshwater molluscs. A short biography is presented; an overview is given of the taxa described by him, as well as a bibliography of his malacological papers. He introduced 119 new taxa, and (co)authored 114 papers over a time span of 55 years. Most of his work is dedicated to former Yugoslavia, Greece, and southeast Asia. His collection of about 30,000 dry samples from 6600 species and 2500 ethanol samples has been included in that of Naturalis Biodiversity Center (Leiden).