Tetradiids are common Ordovician fossils in many parts of the world. Their occurrence in the western interior of North America has been noted, but they have never previously been described from this region. Rhabdotetradium garsonense n.sp. occurs in the Selkirk Member of the Red River Formation at Garson, Manitoba (Late Ordovician, Maysvillian). This species is characterized by small corallites with diameters commonly 0.6–1.1 mm; in transverse sections these are separated from one another or occur in clumps or chains. The variability of corallite arrangement within single coralla of this species illustrates the difficulty of discriminating tetradiid genera. The diagnosis of Rhabdotetradium is revised to more clearly delineate the boundary between it and Paratetradium, which has corallites arranged contiguously in elongate chains.