The mammalian retina maintains high sensitivity over an extraordinary range of luminance levels, ranging from starlight to bright sunlight. This is achieved by switching between the rod and cone systems, and within each system, by employing light adaptation mechanisms that preserve a contrast-invariant response. While dopamine is known to modulate retinal network activity in proportion to luminance, the luminance-dependent signals and the circuits that transmit them to elicit light adaptation, remain elusive. Our work identified a family of luxotonic amacrine cells in which activity increases with luminance level and are poised to modulate retinal function in a luminance-dependent manner. We determine the kinetics of the synaptic input to luxotonic amacrine cells, identify their pre- and postsynaptic partners, and test their role in retinal light adaptation. Implementing optogenetics, chemogenetics, functional imaging, whole-cell electrophysiology, and serial section electron microscopy, this study will help further our understanding of a fundamental adaptative mechanism in a key sensory modality.