The original material of Potamogeton × subrufus Hagstr. [= P. lucens L. × P. nodosus Poir.] and the herbarium material of the morphologically most similar hybrid, P. × fluitans Roth [= P. lucens L. × P. notans L.], were examined taxonomically. Approximately 45 characters were studied on 42 herbarium specimens of both taxa. The analysis shows that P. × subrufus displays several characters consistently different from those of P. × fluitans. Two new European localities of P. × subrufus found during the study are also provided and a morphological description of the hybrid, including distinguishing characters, is given.