IL-10-producing B cells (B10 cells) have been shown to play a suppressive role in the peripheral blood of humans, with their numbers and function altered in several autoimmune diseases. However, the role of B10 cells in Graves' disease (GD) remains unknown. In this study, we demonstrated that B10 cells in human peripheral blood belonged to a CD24(hi)CD27(+) B cell subpopulation. The proportion of B10 cells along with the CD19(+)CD24(hi)CD27(+) B cell subset was significantly lower in new-onset patients compared with healthy individuals. In recovered patients, these proportions were restored to levels similar to those seen in healthy individuals. Additionally, we found that CD19(+)CD24(hi)CD27(+) B cells from healthy individuals inhibited proliferation and TNF-α production of CD4(+) T cells via an IL-10-independent pathway. They also inhibited IFN-γ production by CD4(+) T cells, through an IL-10-dependent pathway. In contrast, their suppressive function on CD4(+) T cell proliferation and cytokine production was impaired in new-onset and recovered patients compared with healthy individuals. Our study provides evidence that CD19(+)CD24(hi)CD27(+) B cells may possess the capacity to downregulate immune responses, partially by production of IL-10 in human peripheral blood. Impairment of their immunosuppressive function may contribute to GD pathogenesis and relapse.