Lateral habenula (LHb) neurons signal negative "reward-prediction errors" and inhibit midbrain dopamine (DA) neurons. Yet LHb neurons are largely glutamatergic, indicating that this inhibition may occur through an intermediate structure. Recent studies in rats have suggested a candidate for this role, the GABAergic rostromedial tegmental nucleus (RMTg), but this neural pathway has not yet been tested directly. We now show using electrophysiology and anatomic tracing that (1) the monkey has an inhibitory structure similar to the rat RMTg; (2) RMTg neurons receive excitatory input from the LHb, exhibit negative reward-prediction errors, and send axonal projections near DA soma; and (3) stimulating this structure inhibits DA neurons. Surprisingly, some RMTg neurons responded to reward cues earlier than the LHb, and carry "state-value" signals not found in DA neurons. Thus, our data suggest that the RMTg translates LHb reward-prediction errors (negative) into DA reward-prediction errors (positive), while transmitting additional motivational signals to non-DA networks.