The ultraviolet radiation (UV) in sunlight can suppress the immune system, which explains why sunlight is such a powerful carcinogen. At the same time, sunlight exposure can protect against some inflammatory diseases, such as multiple sclerosis (MS). This is demonstrated by the latitude-gradient effect, where geographical regions with low UV exposure are associated with higher MS prevalence. Dermatologists have used artificial UV light to control skin inflammation for decades. It is not clear whether UV phototherapy can produce the immunological effects that afford protection against systemic (non-skin) inflammatory diseases. We investigated the immunomodulatory capacity of narrowband UVB (NBUVB), which contains a narrow range of UV (311–312 nm) and is commonly used to treat psoriasis. We examined immunological effects of NBUVB in mice both at the site of irradiation (skin) and in distant tissues (blood, lymph node and spleen). We found that while NBUVB induced some of the local cellular changes characteristic of sunlight exposure (neutrophil infiltration and Langerhans cell depletion), mast cells were surprisingly unaffected. Sunlight can cause systemic immune suppression by disrupting the normal movement of lymphocytes around the body, but NBUVB did not activate this pathway. However, NBUVB effectively suppressed antigen-specific T cell-mediated killing in the spleen. In summary, NBUVB activated some, but not all of the immune-suppressive pathways associated with sunlight exposure. Understanding the wavelength-dependant effects of UV on the immune system will allow us to more effectively harness its immunomodulatory capacity to treat a wider range of diseases.