We quantitatively assessed the association between a vegetarian diet and diabetes risk using pooled estimates from observational studies. Electronic database searches for articles published from January 1980 to May 2016 were independently performed by two investigators, and 13 articles (14 studies) were identified. The pooled odds ratio (OR) for diabetes in vegetarians vs. non-vegetarians was 0.726 (95% confidence interval (CI): 0.608, 0.867). In the subgroup analyses, this inverse association was stronger for the studies conducted in the Western Pacific region (OR 0.514, 95% CI: 0.304, 0.871) and Europe/North America (OR 0.756, 95% CI: 0.589, 0.971) than studies conducted in Southeast Asia (OR 0.888, 95% CI: 0.718, 1.099). No study had a substantial effect on the pooled effect size in the influence analysis, and the Egger's (p = 0.465) and Begg's tests (p = 0.584) revealed no publication bias. This meta-analysis indicates that a vegetarian diet is inversely associated with diabetes risk. Our results support the need for further investigations into the effects of the motivations for vegetarianism, the duration of the adherence to a vegetarian diet, and type of vegetarian on diabetes risk.