Unio rectangularis Tapparone-Canefri, 1889 is a little-known nominal species of freshwater mussels described from a tributary of the Ayeyarwady River in Myanmar. This taxon was considered a synonym of Gibbosula laosensis (Lea, 1863), a margaritiferid species. However, the range of Gibbosula laosensis does not encompass the Ayeyarwady River watershed. Here we re-examine the holotype of Unio rectangularis and provide a conchological re-description of this species. Based on conchological features such as the shell shape, elevated umbo, and the structure of lateral and pseudocardinal teeth, we transfer this taxon to the genus Indonaia Prashad, 1918 and propose I. rectangularis (Tapparone-Canefri, 1889), comb. nov. It appears to be a rare freshwater mussel species with a restricted range, because it has not been found since the original description. Two additional species in this genus are known from Myanmar, i.e. Indonaia andersoniana (Nevill, 1877) and I. subclathrata (Martens, 1899).