Objective: To analyze trends in the prevalence of overweight and obesity among Mongolian adults during the past decade as measured by body mass index (BMI) and waist circumference (WC). Methods: Data from the repeated cross-sectional surveys on the prevalence of noncommunicable disease risk factors conducted in 2005, 2009, and 2013 in Mongolia were used. Linear regression was used to quantify trends in mean BMI and WC, adjusted for age group, sex, and survey year. Results: The age-standardized prevalence of obesity, denoted by the international BMI cutoff values, in men and women between 2005 and 2013 increased from 10.8% to 17.6% and from 18.9% to 26.4%, respectively. Using Asian-specific BMI cutoff values for men and women, the age-standardized prevalence of obesity between 2005 and 2013 increased from 20.0% to 32.8% and 33.4% to 43.7%, respectively. Conclusions: The prevalence of overweight and obesity has increased markedly between 2005 and 2013 similarly across all age groups and sexes. It is important to consider the use of Asian-specific cut-offs as the burden of obesity is twice as high as when using international BMI cutoffs. These data demonstrate the urgent need for obesity treatment, prevention, and monitoring in Mongolia.