Early Paleozoic calcareous algae are potentially useful for stratigraphic correlation but remain underutilized, likely due to presence of graptolites, conodonts, brachiopods, and other fossils that are commonly used in high-resolution biostratigraphy. This study focuses on the siphonous green algae within a 2-to-24-m-thick B interval of the Red River Formation, North Dakota, where the abundance of green algae suggests an important paleoenvironmental control; the algae also had a major role in carbonate production during that narrow stratigraphic interval. The bryopsidalean genus Dimorphosiphon Høeg is abundant in algal wacke-packstone facies interpreted as shallow subtidal deposits. One hundred and twenty-two individual Dimorphosiphon thalli were identified and studied in detail in randomly oriented thin sections; measurements indicate that Williston Basin specimens belong to the species D. talbotorum Boyd, previously reported exclusively from the Bighorn Mountains, Wyoming. Dimorphosiphon is found in Upper Ordovician low-latitude, warm-water shelf carbonates of Kazakhstania, Baltica, and Laurentia; commonly, it is a major component of sediment. Several species of Dimorphosiphon appeared simultaneously in different and remote parts of the Paleotethys and Iapetus Oceans, suggesting a geologically instantaneous dispersal of the genus. Dimorphosiphon talbotorum, the focus of this study, has only been reported from western North America where it occurs within strata corresponding to the upper Katian Aphelognathus divergens Subzone of the Aphelognathus ordovicicus conodont Zone. Given its abundance, ease of identification, and short stratigraphic range, D. talbotorum potentially is very useful for regional correlation, facies, and paleobiogeographic studies of Upper Ordovician Richmondian shallow-marine strata of western North America.