A new, morphologically distinctive, murornate prasinophyte phycoma, Dictyotidium senticogremium sp. nov., is described from Upper Devonian strata of the Geirud Formation, Alborz Mountains, northern Iran. It is characterized by a small, near-circular vesicle, the surface of which is regularly divided by low membranous muri into few relatively large polygonal lacunae; central parts of the latter bear prominent granulate sculpture. Dictyotidium senticogremium sp. nov. occurs in a relatively diverse acritarch, prasinophyte, and miospore assemblage collectively indicating an early Late Devonian (Frasnian) age. The species is persistent, relatively common and has a presumably short stratigraphic range; therefore it is a potentially useful biostratigraphic index fossil for inter-basinal correlation across Iran.