An Iberian semi-slug, Drusia valenciennii (Webb & Van Beneden, 1836), was found in 2019 in the Netherlands. When this introduced species was first encountered, only photographs were taken, but a few juveniles were later col­lected; their raising to maturity is described. One mature specimen was dissected to examine the genitalia and con­firm the identification. This is the first record of a Drusia species in northwestern Europe.