Rationale: B cells contribute to atherosclerosis through subset-specific mechanisms. Whereas some controversy exists about the role of B-2 cells, B-1a cells are atheroprotective because of secretion of atheroprotective IgM antibodies independent of antigen. B-1b cells, a unique subset of B-1 cells that respond specifically to T-cell–independent antigens, have not been studied within the context of atherosclerosis. Objective: To determine whether B-1b cells produce atheroprotective IgM antibodies and function to protect against diet-induced atherosclerosis. Methods and Results: We demonstrate that B-1b cells are sufficient to produce IgM antibodies against oxidation-specific epitopes on low-density lipoprotein both in vitro and in vivo. In addition, we demonstrate that B-1b cells provide atheroprotection after adoptive transfer into B- and T-cell deficient ( Rag1 ^−/− Apoe ^−/− ) hosts. We implicate inhibitor of differentiation 3 (Id3) in the regulation of B-1b cells as B-cell–specific Id3 knockout mice (Id3 ^BKO Apoe ^−/− ) have increased numbers of B-1b cells systemically, increased titers of oxidation-specific epitope–reactive IgM antibodies, and significantly reduced diet-induced atherosclerosis when compared with Id3 ^WT Apoe ^−/− controls. Finally, we report that the presence of a homozygous single nucleotide polymorphism in ID3 in humans that attenuates Id3 function is associated with an increased percentage of circulating B-1 cells and anti–malondialdehyde-low-density lipoprotein IgM suggesting clinical relevance. Conclusions: These results provide novel evidence that B-1b cells produce atheroprotective oxidation-specific epitope–reactive IgM antibodies and protect against atherosclerosis in mice and suggest that similar mechanisms may occur in humans.