Visual attention enhances the neural representation of salient stimuli within the visual cortex. Although it is generally thought that this enhancement is driven by glutamatergic feedback from frontal cortical areas, cortically released neuromodulators are also believed to play a major uncharacterized role. Here we report the unexpected observation that dorsal raphe (DR) derived serotonin (5HT) controls visual attention. We adopted a behavioral model that captured how mice allocated attention to cued and uncued visual features. Simultaneous photometry recordings showed that DR activity decreased when mice deployed attention to the cued features, whereas high DR activity was observed when mice were less attentive. Optogenetic excitation of DR-5HT neurons impaired attention to the cue and degraded behavioral performance, whereas optogenetic suppression improved attention and performance. A genetically encoded sensor of 5HT release showed reduced 5HT levels in visual cortex when mice attend and detect stimuli. These results demonstrate that DR-5HT neurons are members of the brain’s attentional circuit and suggest that 5HT is a novel biological carrier of visual attention.