Abstract The NADPH oxidase of myeloid cells, NOX2, generates reactive oxygen species (ROS) to eliminate pathogens and malignant cells. NOX2-derived ROS have also been proposed to dampen functions of natural killer (NK) cells and other antineoplastic lymphocytes in the microenvironment of established tumors. The mechanisms by which NOX2 and ROS influence the process of distant metastasis have only been partially explored. Here, we utilized genetically NOX2-deficient mice and pharmacologic inhibition of NOX2 to elucidate the role of NOX2 for the hematogenous metastasis of melanoma cells. After intravenous inoculation of B16F1 or B16F10 cells, lung metastasis formation was reduced in B6.129S6-Cybbtm1DinK (Nox2-KO) versus Nox2-sufficient wild-type (WT) mice. Systemic treatment with the NOX2-inhibitor histamine dihydrochloride (HDC) reduced melanoma metastasis and enhanced the infiltration of IFNγ-producing NK cells into lungs of WT but not of Nox2-KO mice. IFNγ-deficient B6.129S7-Ifngtm1Ts/J mice were prone to develop melanoma metastases and did not respond to in vivo treatment with HDC. We propose that NOX2-derived ROS facilitate metastasis of melanoma cells by downmodulating NK-cell function and that inhibition of NOX2 may restore IFNγ-dependent, NK cell–mediated clearance of melanoma cells. Cancer Immunol Res; 5(9); 804–11. ©2017 AACR.