<p>Premature ovarian insufficiency (POI) is one of the most common diseases contributing to declining fertility globally. Autoimmune dysregulation has been implicated in POI, but the role of B cell and its subsets in immune dysregulation in POI remains unclear. In the current study, we investigated whether disturbances in the distribution and functional characteristics of peripheral B cell subsets are associated with POI and its severity. Our results revealed that POI patients exhibited a higher percentage of total CD19⁺ B cells, but the proportion of memory B cells (MBCs) and plasmablasts was significantly reduced compared to controls. A notable decrease in interleukin-10 (IL-10) production by B cells was observed, primarily due to a reduction in CD19⁺CD24^hiCD27⁺ and CD19⁺CD24^hiCD38^hi regulatory B cells (B_regs). These alterations correlated with elevated follicle-stimulating hormone (FSH) and decreased anti-Müllerian hormone (AMH) and antral follicle count (AFC) levels. However, no significant differences were observed in the suppressive capacity of B_regs on interferon-gamma (IFN-γ) or tumor necrosis factor-alpha (TNF-α) production by autologous CD4⁺ T cells in POI patients compared with control women. These findings suggest that B-cell dysregulation may contribute to the immune pathogenesis of POI, providing potential targets for therapeutic intervention.</p>