Terrestrial gastropod inventories can be improved, both in scope and thoroughness, by including species observations made by citizen scientists. Few citizen science projects, however, focus on terrestrial gastropods and perhaps none has mobilized members of the public to survey the malacofauna of a major North American metropolitan area. Here we report first occurrence records of five introduced terrestrial gastropod species in the metropolitan areas of Los Angeles, Orange, and Riverside counties in California, discovered by citizen science: Arion hortensis Férussac, 1819, Cochlicella barbara (Linnaeus, 1758), Lauria cylindracea (Da Costa, 1778), Pupoides albilabris (C.B. Adams, 1841), and Xerotricha conspurcata (Draparnaud, 1801). Four of these taxa are known elsewhere in California and one, L. cylindracea, is a first occurrence record for the U.S.A. All were contributed to SLIME, a citizen science project and malacofaunal inventory of southern California initiated by the Natural History Museum of Los Angeles County and hosted online by iNaturalist. Species identifications were made based on snail or slug morphology and collected specimens' COI barcoding sequences, which were compared to those in GenBank and BOLD databases. These discoveries demonstrate the efficacy of SLIME and the potential for molluscan-focused citizen science to detect and document land snail and slug taxa in a major metropolis.