The habenula integrates sensory stimuli and reward information to regulate the release of neuromodulators with broad effects on brain state and behavior. One stimulus that affects habenula activity is light, but how it does so is unknown. Here, we address this question using larval zebrafish. Calcium imaging shows that light evokes widespread activity in habenula neurons, coupled with a prominent early response in the dorsal left neuropil. Injection of a lipophilic dye into this region retrogradely labels a retino-recipient thalamic nucleus. Anterograde tracing of the thalamus demonstrates a projection to the habenula, while optogenetic and lesion experiments confirm functional connectivity. An analysis of the mouse mesoscale connectome indicates that a visual nucleus in the thalamus, the ventral lateral geniculate nucleus, projects to the habenula in this species also. Together, these data suggest the existence of a conserved thalamo-habenula projection that enables light to affect habenula activity in vertebrates.