The Ras small GTPase functions as a signaling node and is activated by extracellular stimuli. Upon activation, Ras interacts with a spectrum of functionally diverse downstream effectors and stimulates multiple cytoplasmic signaling cascades that regulate cellular proliferation, differentiation, and apoptosis. In addition to the association of Ras with the plasma membrane, recent studies have established an association of Ras with Golgi membranes. Whereas the effectors of signal transduction by activated, plasma membrane-localized Ras are well characterized, very little is known about the effectors used by Golgi-localized Ras. In this study, we report the identification of a novel Ras-interacting protein, Rain, that may serve as an effector for endomembrane-associated Ras. Rain does not share significant sequence similarity with any known mammalian proteins, but contains a Ras-associating domain that is found in RalGDS, AF-6, and other characterized Ras effectors. Rain interacts with Ras in a GTP-dependent manner in vitro and in vivo, requires an intact Ras core effector-binding domain for this interaction, and thus fits the definition of a Ras effector. Unlike other Ras effectors, however, Rain is localized to perinuclear, juxta-Golgi vesicles in intact cells and is recruited to the Golgi by activated Ras. Finally, we found that Rain cooperates with activated Raf and causes synergistic transformation of NIH3T3 cells. Taken together, these observations support a role for Rain as a novel protein that can serve as an effector of endomembrane-localized Ras.