IMPORTANCE: The incidence of diverticulitis has been associated with geographic and seasonal variation. Low levels of circulating vitamin D are associated with diverticulitis. We investigated the association between UV light and diverticulitis. OBSERVATIONS: We identified nonelective diverticulitis admissions in the Nationwide Inpatient Sample and linked hospital locations to UV data. We examined UV exposure in relation to risk of admission for diverticulitis. We identified geographic and seasonal trends among 226 522 nonelective admissions for diverticulitis. Compared with high-UV areas, low-UV areas had a higher rate of diverticulitis (751.8 vs 668.1 per 100 000 admissions; P < .001), diverticular abscess (12.0% vs 9.7%; P < .001), and colectomy (13.5%vs 11.5%; P < .001). We also observed significant seasonal variation, with a lower rate of diverticulitis in the winter (645 per 100 000) compared with the summer (748 per 100 000) (P < .001). The summer increase was more evident in areas with the greatest UV fluctuation vs areas with the least UV fluctuation (120 vs 70 per 100 000; P = .01). CONCLUSIONS AND RELEVANCE: Low UV light exposure is associated with an increased rate of diverticulitis admissions and greater seasonal variation. Because UV exposure largely determines vitamin D status, these findings support a role for vitamin D in the pathogenesis of diverticulitis.