Evaluation of infarct volumes and infiltrating immune cell populations in mice after middle cerebral artery occlusion (MCAO) strongly implicates a mixture of both pathogenic and regulatory immune cell subsets in stroke pathogenesis and recovery. Our goal was to evaluate the contribution of B cells to the development of MCAO by comparing infarct volumes and functional outcomes in wild-type (WT) versus B-cell-deficient μMT^−/− mice. The results clearly demonstrate larger infarct volumes, higher mortality, more severe functional deficits, and increased numbers of activated T cells, macrophages, microglial cells, and neutrophils in the affected brain hemisphere of MCAO-treated μMT^−/− versus WT mice. These MCAO-induced changes were completely prevented in B-cell-restored μMT^−/− mice after transfer of highly purified WT GFP⁺ B cells that were detected in the periphery, but not the CNS. In contrast, transfer of B cells from IL-10^−/− mice had no effect on infarct volume when transferred into μMT^−/− mice. These findings strongly support a previously unrecognized activity of IL-10-secreting WT B cells to limit infarct volume, mortality rate, recruitment of inflammatory cells, and functional neurological deficits 48 h after MCAO. Our novel observations are the first to implicate IL-10-secreting B cells as a major regulatory cell type in stroke and suggest that enhancement of regulatory B cells might have application as a novel therapy for this devastating neurologic condition.