Following T-cell receptor and CD28 signaling, CD8+ T cells express a receptor for CD83, a molecule up-regulated on functionally mature dendritic cells. Although this expression pattern suggests that CD83 is involved in adaptive immunity, little is known about its function in the periphery, and the existence of its ligand on T cells is controversial. We demonstrate that the engagement of the CD83 ligand (CD83L) preferentially enriches and significantly amplifies the number of antigen-specific CD8+ T cells. Coengagement of the T-cell receptor, CD28, and CD83L supports priming of naive CD8+ T cells that retain antigen specificity and cytotoxic function for more than 6 months. Therefore, engagement of the CD83L provides a unique signal to activated CD8+ T cells that could be exploited to generate long-lived antigen-specific cytotoxic T cells for the treatment of cancer and infection.