Recent experimental and epidemiologic studies have suggested air pollution as a new risk factor for type 2 diabetes mellitus (T2DM). We conducted a systematic review of the epidemiologic studies on the association of air pollution with T2DM and related outcomes published by December 2013. We identified 22 studies: 6 prospective studies on incident T2DM; 2 prospective study on diabetes mortality; 4 cross-sectional studies on prevalent T2DM; 7 ecological studies on mortality or morbidity from diabetes; and 3 studies on glucose or insulin levels. The evidence of the association between long-term exposure to fine particles (PM2.5) and the risk of T2DM is suggestive. The summary hazard ratio of the association between long-term PM2.5 exposure and incident T2DM was 1.11 (95% CI, 1.03-1.19) for a 10 μg/m3 increase. The evidence on the association between long-term traffic-related exposure (measured by nitrogen dioxide or nitrogen oxides) and the risk of T2DM was also suggestive although most studies were conducted in women. For short-term effects of air pollution on diabetes mortality or hospital/emergency admissions, we conclude that the evidence is not sufficient to infer a causal relationship. Because most studies were conducted in North America or in Europe where exposure levels are relatively low, more studies are needed in recently urbanized areas in Asia and Latin America where air pollution levels are much higher and T2DM is an emerging public health concern.