How light therapy ameliorates depression is not well understood. Here, tracing techniques revealed that the circuit responsible for this effect involves M4-type retinal ganglion cells (RGCs) that project via a disynaptic circuit onto GABAergic neurons in the thalamic ventral lateral geniculate nucleus and intergeniculate leaflet (vLGN/IGL). Projections of these neurons in turn inhibit excitatory neurons of the lateral habenula (LHb). Depressive-like behaviours evoked by chronic exposure to aversive stimuli or social defeat stress were attenuated following activation of this RGC–vLGN/IGL–LHb pathway or by visual light.