Despite the frequent occurrence of hybridization events between broad-leaved members of the genus Potamogeton, few documented reports and descriptions of these taxa are found in North American pondweed literature. Potamogeton crispus and P. praelongus are common broad-leaved pondweed species with overlapping distributions in North America, yet no confirmed report of their hybrid, P. × undulatus, has been previously documented from this continent. We provide the first confirmed report of P. × undulatus in North America, where it was collected from a northeastern Indiana lake. Because most descriptions of P. × undulatus are found in somewhat less accessible nineteenth century European pondweed literature, morphological characters that are diagnostically valuable in distinguishing the hybrid from its parents are discussed and illustrated. A vegetative key to this hybrid and its parental species is also provided.