The vestibular system broadcasts head-movement-related signals to sensory areas throughout the brain, including visual cortex. These signals are crucial for the brain's ability to assess whether motion of the visual scene results from the animal's head movements. However, how head movements affect visual cortical circuits remains poorly understood. Here, we discover that ambient luminance profoundly transforms how mouse primary visual cortex (V1) processes head movements. While in darkness, head movements result in overall suppression of neuronal activity; in ambient light, the same head movements trigger excitation across all cortical layers. This light-dependent switch in how V1 processes head movements is controlled by somatostatin-expressing (SOM) inhibitory neurons, which are excited by head movements in dark, but not in light. This study thus reveals a light-dependent switch in the response of V1 to head movements and identifies a circuit in which SOM cells are key integrators of vestibular and luminance signals.