Herpesviruses infect a wide range of vertebrates, including toothed whales of the order Cetacea. One of the smallest toothed whales is the harbour porpoise (Phocoena phocoena), which is widespread in the coastal waters of the northern hemisphere, including the North Sea. Here, we describe the detection and phylogenetic analysis of a novel gammaherpesvirus associated with mucocutaneous and skin lesions in stranded harbour porpoises along the Dutch coast, tentatively designated phocoenid herpesvirus 1 (PhoHV1). Phylogenetically, PhoHV1 forms a monophyletic clade with all other gammaherpesviruses described in toothed whales (Odontoceti) to date, suggesting a common evolutionary origin.