Some flagellated bacteria regulate motor torque in response to the external load change. This behavior is critical for survival, but the mechanism has remained unknown. Here, we focused on a key protein, FliL of Vibrio alginolyticus , and solved the crystal structure of its periplasmic region (FliL _Peri ). FliL _Peri reveals striking structural similarity to a conserved domain of stomatin, which is involved in ion channel regulation in some organisms, including mammals. FliL _Peri forms a ring with an inner diameter that is comparable in size to the stator unit. The mutational analyses suggested that the presence of the ring-like assembly of FliL around the stator unit enhances the surface swarming of Vibrio cells. Our study data also imply that the structural element for the ion channel regulation is conserved from bacteria to mammals.