Palynological investigations of Pliocene–Pleistocene sediments from Ocean Drilling Program (ODP) Holes 882A and 887C, located in the western and eastern North Pacific respectively, have revealed the occurrence of two new dinoflagellate cyst species of the genus Impagidinium Stover and Evitt, 1978. Impagidinium detroitense sp. nov. has a spheroidal to broadly ovoidal central body and finely granulate surface with suturocavate septa that are higher antapically than apically. This species is recorded from the Lower to Upper Pliocene (5.27–2.70 Ma), with a maximum abundance at 3.95–3.58 Ma and a disappearance possibly coinciding with the onset of the modern halocline in the North Pacific at 2.7 Ma. Impagidinium? diaphanum sp. nov. is a large and thin-walled species with nearly complete tabulation expressed by low and pronounced cavate septa, and an unusual ventral tabulation in which the anterior sulcal plate (as) contacts the fifth precingular plate (5″). Impagidinium? diaphanum sp. nov. occurs within short, non-synchronous time windows during the Early Pliocene. These two new species have not been reported previously, suggesting regional endemism during the Pliocene. The stratigraphically well-defined occurrence of these two species highlights the potential of organic-walled dinoflagellate cysts for biostratigraphic applications at local to regional scales in the subarctic North Pacific.