A new protoperidiniacean dinoflagellate cyst species recorded from the Lower Pliocene Serripes Zone of the Tjörnes beds in northern Iceland is formally described and its palaeoecological preferences are evaluated. Selenopemphix islandensis sp. nov. is a thin-walled cyst with strong polar compression and a reniform outline in apical view. The wide cingular margins are dorsally ornamented with processes of varying morphology and ventrally with large fanshaped processes. A distinct size difference between the dorsal and ventral cingular processes is a major diagnostic characteristic of this species. A differential diagnosis for Selenopemphix islandensis sp. nov. is presented. Within the Tjörnes section, Selenopemphix islandensis sp. nov. is restricted to the uppermost Serripes Zone of the Tjörnes beds. The base of this zone corresponds to the invasion of cold-water molluscs of Pacific affinity through the Bering Strait into the northern Atlantic. The sudden appearance of Selenopemphix islandensis sp. nov. from this level can possibly be related to this northern migration through the Bering Strait, and suggest a Pacific origin. The new species is not recorded in the underlying Lower Pliocene Tapes Zone and Mactra Zone of the Tjörnes beds, and is also absent in the superjacent Pleistocene Breidavík Group. Selenopemphix islandensis sp. nov. has a first and last appearance in the Tjörnes region at c. 4.5 Ma and 4.2 Ma, respectively. Palaeoecological studies on molluscs, ostracods, plants and oxygen isotopes indicate an average summer temperature of 5–10^°C for the Serripes Zone, which is comparable to the present-day situation in northern Iceland.