Significance The increasing prevalence of myopia is a significant public health concern. Unfortunately, the mechanisms driving myopia remain elusive, limiting effective treatment options. This report identifies a refractive development pathway that requires Opn5 -expressing retinal ganglion cells (RGCs). Stimulation of Opn5 RGCs with short-wavelength violet light prevented experimental myopia in mice. Furthermore, this effect was dependent on the time of day, with evening exposure being sufficient to protect against experimental myopia. Thus, these studies suggest Opn5 RGCs may contribute to the mechanisms of emmetropization and identify the OPN5 pathway as a potential target for the treatment of myopia. Myopia has become a major public health concern, particularly across much of Asia. It has been shown in multiple studies that outdoor activity has a protective effect on myopia. Recent reports have shown that short-wavelength visible violet light is the component of sunlight that appears to play an important role in preventing myopia progression in mice, chicks, and humans. The mechanism underlying this effect has not been understood. Here, we show that violet light prevents lens defocus–induced myopia in mice. This violet light effect was dependent on both time of day and retinal expression of the violet light sensitive atypical opsin, neuropsin (OPN5). These findings identify Opn5 -expressing retinal ganglion cells as crucial for emmetropization in mice and suggest a strategy for myopia prevention in humans.