Immunoglobulin M (IgM) is the first antibody produced during embryonic development and the humoral immune response. IgM can take various forms, such as membrane-bound IgM (mIgM) IgM within the B-cell receptor (BCR) complex, pentameric and hexameric IgM in serum, and secretory IgM (SIgM) on mucosal surfaces. FcμR/Toso/Faim3, the sole receptor specific to IgM in mammals, plays a role in regulating diverse immune responses by recognizing different forms of IgM. We demonstrate that two FcμR molecules interact with a Cμ4 dimer, indicating that FcμR can bind to mIgM with a 2:1 stoichiometry. Further analysis reveals that FcμR-binding sites are accessible when IgM is in the context of the IgM-BCR complex. Additionally, we show that FcμR can form a 4:1 complex with pentameric IgM. Notably, all four FcμR molecules bind to the same side, suggesting that pentameric IgM can facilitate the formation of an FcμR oligomer. In contrast, four FcμR molecules bind to the opposite side of SIgM, which contains the secretory component. These findings represent significant progress in understanding the effector functions of IgM.