Tanning bed users have a significantly increased risk of melanoma, but it remains unclear how indoor tanning drives melanomagenesis. To better understand the etiology of melanoma associated with tanning bed use, we described the patterns of melanoma in patients with quantifiable tanning bed usage and performed exome sequencing of melanocytes from normal skin of a subset of these patients. Tanning bed users were more likely to have melanoma on body sites with low cumulative levels of sun damage and to have multiple melanomas. The melanocytes in skin from tanning bed users had higher mutation burdens and higher proportions of cells with pathogenic mutations—these differences were most prominent over body sites that experience comparatively less exposure to natural sunlight. We conclude that tanning bed radiation induces melanoma by increasing the mutation burden of melanocytes and by mutagenizing a broader field of melanocytes than are typically exposed to natural sunlight.