A first record of Wolffia columbiana from Belgium. – In August 2017, during field work in a private area adjacent to the Grootbroek forest reserve in Kinrooi (province of Limburg), a vegetation mass of an unknown exotic aquatic plant was discovered. Closer examination revealed that it belonged to the American invasive alien species Wolffia columbiana, a species not yet reported from Belgium. Soon afterwards a second population was found in the same area. In this paper some details about these localities are provided and an overview of Wolffia in Europe, emphasizing on distribution and recognition, is presented.