Systemic lupus erythematosus (SLE) is a systemic autoimmune disease characterized by a loss of tolerance toward self-nucleic acids, autoantibody production, interferon expression and signaling, and a defect in the regulatory T (T_reg) cell compartment. In this work, we identified that platelets from patients with active SLE preferentially interacted with T_reg cells via the P-selectin/P-selectin glycoprotein ligand-1 (PSGL-1) axis. Selectin interaction with PSGL-1 blocked the regulatory and suppressive properties of T_reg cells and particularly follicular T_reg cells by triggering Syk phosphorylation and an increase in intracytosolic calcium. Mechanistically, P-selectin engagement on T_reg cells induced a down-regulation of the transforming growth factor–β axis, altering the phenotype of T_reg cells and limiting their immunosuppressive responses. In patients with SLE, we found an up-regulation of P- and E-selectin both on microparticles and in their soluble forms that correlated with disease activity. Last, blocking P-selectin in a mouse model of SLE improved cardinal features of the disease, such as anti-dsDNA antibody concentrations and kidney pathology. Overall, our results identify a P-selectin–dependent pathway that is active in patients with SLE and validate it as a potential therapeutic avenue.