A unique subpopulation of spleen dendritic cells (DCs) that express the CD8 surface marker efficiently present phagocytosed antigens to CD8⁺ T lymphocytes in a process called "crosspresentation," which initiates cytotoxic immune responses. We now show that the small GTPase Rac2 plays a critical role in antigen crosspresentation selectively in this DC subpopulation. In CD8⁺ DCs, Rac2 determines the subcellular assembly of the NADPH oxidase complex (NOX2) to phagosomes, whereas in CD8⁻ DCs, Rac1 mediates the assembly of NOX2 at the plasma membrane. In the absence of Rac2, the production of reactive oxygen species (ROS) in DC-phagosomes was abolished, the phagosomal pH dropped, and the efficiency of antigen crosspresentation was reduced. We conclude that the activity of Rac1 and 2 control crosspresentation in DC subpopulations through the regulation of phagosomal oxidation and pH.