Natural immunoglobulin M (IgM) is reactive to autoantigens and is believed to be important for autoimmunity. Blood pentameric IgM loaded with antigens forms a large immune complex (IC) that contains various elements, including apoptosis inhibitor of macrophage (AIM). Here we demonstrate that this IgM-AIM association contributes to autoantibody production under obese conditions. In mice fed a high-fat diet, natural IgM increased through B cell TLR4 stimulation. AIM associated with IgM and protected AIM from renal excretion, increasing blood AIM levels along with the obesity-induced IgM augmentation. Meanwhile, the AIM association inhibited IgM binding to the Fcα/μ receptor on splenic follicular dendritic cells, thereby protecting the IgM IC from Fcα/μ receptor-mediated internalization. This supported IgM-dependent autoantigen presentation to B cells, stimulating IgG autoantibody production. Accordingly, in obese AIM-deficient (AIM−/−) mice, the increase of multiple IgG autoantibodies observed in obese wild-type mice was abrogated. Thus, the AIM-IgM association plays a critical role in the obesity-associated autoimmune process. Display omitted ► AIM is protected from renal excretion via binding to IgM pentamers in blood ► Association with AIM interferes with Fcα/μ receptor-mediated internalization of IgM ► IgM-dependent autoantigen presentation on FDCs is preserved by AIM ► Absence of AIM tempers obesity-associated multiple IgG autoantibody production In addition to inducing metabolic and cardiovascular diseases, obesity also increases blood immunoglobulin M (IgM) levels, followed by multiple autoantibody expansion leading to autoimmune disease. Here, Arai, Miyazaki, and colleagues show that apoptosis inhibitor of macrophage (AIM) associates with IgM and abrogates IgM internalization through the Fcα/μ receptor. This response supports IgM-dependent autoantigen presentation to B cells on splenic follicular dendritic cells, thereby enhancing the development of high-affinity IgG autoantibody-producing plasma cells. Thus, AIM suppression may be a promising candidate for the next generation of therapy for preventing obesity-associated autoimmune diseases.