Background: The vegetarian diet continues to gain recognition and popularity among people; however, few studies have considered the level of knowledge of professional dietitians about this dietary pattern. Objective: This study aimed to compare the level of knowledge of vegetarian and nonvegetarian Peruvian dietitians regarding vegetarianism at different life stages. Methods: A cross-sectional study was carried out. An online questionnaire based on the recommendations of the current dietary guidelines was administered to more than 400 registered dietitians. Of which, a total of 179 decided to participate in the study: 72 vegetarians and 107 nonvegetarians. The data were analyzed using the chi-square test, considering a significance level of 5%. Results: Women represented the largest proportion of the sample. The participants demonstrated a complete and exhaustive knowledge of the definition of vegetarian diets. Regarding the risks and benefits associated with vegetarianism, the largest proportion of those who got the correct answers were vegetarians. The percentage of correct answers selected for both groups regarding the critical nutrients were less than 50%. Only 17.6% identified the correct answer regarding the risk of eating disorders of vegetarianism. Conclusions: Dietitians did not demonstrate complete and comprehensive knowledge of the critical nutrients of vegetarianism and lack information on the risks of eating disorders from the vegetarian diet.