Invasive Oriental Weatherfish (Misgurnus anguillicaudatus) populations have become established in many countries worldwide. To understand the invasive potential of this non-indigenous species, information about its demography and life history within these novel habitats is needed. Here we present data supporting the use of pectoral fin morphology for determining sex within a population of weatherfish in and around Boise, Idaho, USA. Enlargement of the second pectoral-fin ray and presence of a lamina circularis was seen in males both mature and immature. No such morphology was seen in females. Examination of gonadal tissue confirmed external sex determination in 99% of mature females, 97% of mature males, and 70% of juveniles less than 10 cm total length. This study is the first to verify use of sexually dimorphic fin characteristics as a field diagnostic in an invasive population of Oriental Weatherfish.