Memory CD8⁺ T cells are critical for host defense upon reexposure to intracellular pathogens. We found that interleukin 10 (IL-10) derived from CD4⁺ regulatory T cells (T_reg cells) was necessary for the maturation of memory CD8⁺ T cells following acute infection with lymphocytic choriomeningitis virus (LCMV). T_reg cell–derived IL-10 was most important during the resolution phase, calming inflammation and the activation state of dendritic cells. Adoptive transfer of IL-10-sufficient T_reg cells during the resolution phase 'restored' the maturation of memory CD8⁺ T cells in IL-10-deficient mice. Our data indicate that T_reg cell–derived IL-10 is needed to insulate CD8⁺ T cells from inflammatory signals, and reveal that the resolution phase of infection is a critical period that influences the quality and function of developing memory CD8⁺ T cells.