Dendronotus patricki, sp. nov. is a new species collected from a whalefall in the Monterey Canyon, California. This new species is characterized by having a small number of dorsal appendages compared to similarly sized species of Dendronotus. Anatomically, D. patricki, sp. nov. has a small prostate with just a few alveoli, a very small seminal receptacle, situated near the distal end of the vagina, and a relatively short and small ampulla. The rachidian radular teeth of D. patricki, sp. nov. are unique among Dendronotus as they have a well-differentiated, conical cusp with very small denticles on either side, but most denticles are located on the sides of the teeth, rather than on the sides of the cusp. Dendronotus patricki, sp. nov, is genetically distinct from other species of Dendronotus for which sequence data are available. A phylogenetic analysis of Dendronotus based on COI, 16S, and H3 sequence data reveals that D. patricki, sp. nov. forms a polytomy with Dendronotus orientalis and a clade of the shallow temperate and cold water species. The tropical Indo-Pacific species D. regius is the sister group to all other Dendronotus species.