The bacterial flagellum is a complex nanomachine that allows bacteria to swim. Though the number and structure of this nanomachine varies across species, all bacterial flagella have a long tail-like filament that is secreted by an export apparatus and then rotated by a motor. The bacterial flagellar motor consists of multiple stator units and two ring complexes, the MS-ring and C-ring. The MS-ring spans the inner membrane and interacts with the (conserved) export apparatus and (highly-variable) cytoplasmic C-ring. Despite decades of study, the three-dimensional structure of the MS-ring has remained poorly resolved. Using single-particle cryo-electron microscopy we have obtained high-resolution structures of the MS-ring from three different bacterial species. These structures reveal how the MS-ring uses an unprecedented level of symmetry variation to function as an adaptor between the export apparatus and the C-ring. Our findings also give insight into how the MS-ring varies across different bacterial species to accommodate observed differences in the C-ring.