The acritarch genus Orthosphaeridium Eisenack 1968 is one of the most frequently recorded acritarch taxa in the Ordovician. The taxonomy of this easily recognizable genus is revised herein based on a detailed literature review and on new investigations of palynological material from northeastern Iran and South China. The review confirms that Baltisphaera Burmann 1970 is a junior synonym of Orthosphaeridium of which the diagnosis is here emended. Over 20 species have been described in the literature, but many of them are apparently synonyms. The review indicates that the following four species can be easily differentiated: Orthosphaeridium bispinosum Turner 1984, Orthosphaeridium ternatum (Burmann 1970) Eisenack et al. 1976, Orthosphaeridium rectangulare (Eisenack 1963) Eisenack 1968 (type species) and Orthosphaeridium octospinosum Eisenack 1968. The taxonomical ranks of Orthosphaeridium rectangulare var. quadricornis Burmann 1970 and Orthosphaeridium octospinosum var. insculptum Loeblich 1970 are changed from the species to the varietas level. The review indicates that Orthosphaeridium first appeared in South China in the Expansograptus hirundo graptolite Biozone, i.e. in the early Dapingian (stage slice Dp1 and time slice 3a of the Middle Ordovician) and that it reached a global distribution during the Middle and Late Ordovician. The genus disappeared at the end of the Ordovician and has never been recorded from the Silurian. The genus is thus a stratigraphical index fossil for the Middle and Late Ordovician. Orthosphaeridium has been recorded from the margins of all paleocontinents and paleoecologically seems to be an eurythermic taxon. (C) 2019 Elsevier B.V. All rights reserved.