Galectin-3 is a β-galactoside-binding protein with an inhibitory role in B cell differentiation into plasma cells in distinct lymphoid tissues. We use a model of chronic schistosomiasis, a well-characterized experimental disease hallmarked by polyclonal B cell activation, in order to investigate the role of galectin-3 in controlling IgA production through peritoneal B1 cells. Chronically infected, galectin-3-deficient mice (Lgals3 −/−) display peritoneal fluid hypercellularity, increased numbers of atypical peritoneal IgM+/IgA+ B1a and B1b lymphocytes and histological disturbances in plasma cell niches when compared with Lgals3 +/+ mice. Similar to our infection model, peritoneal B1 cells from uninfected Lgals3 −/− mice show enhanced switching to IgA after in vitro treatment with interleukin-5 plus transforming growth factor-β (IL-5 + TGF-β1). A higher number of IgA+ B1a lymphocytes was found in the peritoneal cavity of Lgals3 −/−-uninfected mice at 1 week after i.p. injection of IL-5 + TGF-β1; this correlates with the increased levels of secreted IgA detected in the peritoneal fluid of these mice after cytokine treatment. Interestingly, a higher number of degranulated mast cells is present in the peritoneal cavity of uninfected and Schistosoma mansoni-infected Lgals3 −/− mice, indicating that, at least in part, mast cells account for the enhanced differentiation of B1 into IgA-producing B cells found in the absence of galectin-3. Thus, a novel role is revealed for galectin-3 in controlling the expression of surface IgA by peritoneal B1 lymphocytes; this might have important implications for manipulating the mucosal immune response.