Context: Vegetarian and vegan diets are increasing in popularity. Although they provide beneficial health effects, they may also lead to nutritional deficiencies. Cognitive impairment and mental health disorders have a high economic burden. Objective: A meta-analysis was conducted to examine the relationship between vegan or vegetarian diets and cognitive and mental health. Data Sources: PubMed, Scopus, ScienceDirect, and Proquest databases were examined from inception to July 2018. Study Selection: Original observational or interventional human studies of vegan/vegetarian diets were selected independently by 2 authors. Data Extraction: Raw means and standard deviations were used as continuous outcomes, while numbers of events were used as categorical outcomes. Results: Of 1249 publications identified, 13 were included, with 17 809 individuals in total. No significant association was found between diet and the continuous depression score, stress, well-being, or cognitive impairment. Vegans/vegetarians were at increased risk for depression (odds ratio = 2.142; 95%CI, 1.105-4.148) and had lower anxiety scores (mean difference = -0.847; 95%CI, -1.677 to -0.018). Heterogeneity was large, and thus subgroup analyses showed numerous differences. Conclusions: Vegan or vegetarian diets were related to a higher risk of depression and lower anxiety scores, but no differences for other outcomes were found. Subgroup analyses of anxiety showed a higher risk of anxiety, mainly in participants under 26 years of age and in studies with a higher quality. More studies with better overall quality are needed to make clear positive or negative associations.