We report the first archived collection of the invasive obligate wetland monocot Butomus umbellatus in the Allegheny River, a major tributary to the Ohio River. The specimen we discovered represents the first formally collected specimen in Pennsylvania outside of the Lake Erie watershed. Our collection complements multiple observations on iNaturalist throughout the larger Ohio River watershed that collectively suggest a recent, rapid expansion of the species southward. Such observations highlight the potential to rapidly identify novel or advancing invasive species using citizen science prior to recognition in formal herbarium collections and associated databases.