Apocynin is known to suppress the production of reactive oxygen species (ROS) by inhibiting NADPH oxidases, specifically phagocytic NADPH oxidase (PHOX or NOX2). Given the pro-inflammatory effects of ROS, apocynin has been studied extensively for its use as a therapeutic agent in various disease models. While the effects of apocynin on neutrophils and monocytes have been investigated, it remains to be elucidated whether apocynin modulates the effector function of T cells. In the present study, we examined the effect of apocynin on CD8 + T cells and further investigated its mechanism of action. We found that apocynin directly inhibited the production of pro-inflammatory cytokines such as TNF-α, IFN-γ, and IL-2 in anti-CD3/anti-CD28-stimulated CD8+ T cells. The action of apocynin was upstream of the protein kinase C and calcium signaling in the T cell receptor signaling pathway because apocynin did not inhibit cytokine production in phorbol 12-myristate 13-acetate/ionomycin-stimulated CD8+ T cells. Electrophoretic mobility shift assays revealed that apocynin attenuated anti-CD3/anti-CD28- induced NF-κB activation in CD8+ T cells. In the experiments with NOX2-deficient mice, we demonstrated that apocynin inhibited TNF-α production of CD8+ T cells in a NOX2-independent manner. Taken together, we demonstrated that apocynin, a well-known NOX2 inhibitor, suppressed the cytokine production of CD8+ T cells. We also showed the NOX2-independent action of apocynin in the inhibition of TNF-α production in CD8+ T cells.