The opisthobranch fauna of the SE Kamchatka peninsula (NW Pacific, Russia) has been intensively investigated for the first time resulting in 21 species found, and six new records for Kamchatka. A unique onchidoridid species with a well defined, closeable gill pocket into which gills can be fully retracted, and which does not fit with any existing genus is described as Onchimira cavifera gen. et sp. nov. in anatomical detail. Onchimira cavifera is remarkable in that it combines characteristic features of cryptobranch dorids with those of phanerobranch onchidoridids. Its role as a missing link and new evidence for evolutionary reduction of the gill cavity within phanerobranch dorid lineages is discussed. In addition, two new species of the genus Adalaria, A. slavi sp. nov. and A. olgae sp. nov., are described based on numerous newly collected specimens showing little intraspecific variations, but clear differences to all previously described Adalaria species. A review of the presently known Adalaria species including SEM data of the radula and labial cuticle is presented in order to highlight differences among the newly discovered taxa. A new species of the nudibranch genus Onchidoris, O. macropompa sp. nov., is described. A comparative table of all presently known Onchidoris species is presented.