Background: Obesity in young adults is rising and predicts diabetes and cardiovascular diseases later in life. Data on prevalence and determinants of obesity in developing countries are needed for primary prevention. Objectives: To determine the prevalence of overweight and obesity in young adults in urban (Kampala city) and rural areas (Kamuli District) of Uganda. Methods: Cross-sectional survey of 683 randomly selected young adults aged 18-30 years. Obesity was defined as body mass index (BMI) > 30 kg/m2 and overweight as BMI > 25 kg/m2. Distribution of BMI by socio-demographic characteristics was determined. Results: Of the 683 participants, 50.5% were female and 53.2% were from Kampala. The overall prevalence of obesity and overweight was 2.3% and 10.4%, respectively. The prevalence of obesity was 4.4% in Kampala and 0% in Kamuli while the prevalence of overweight was 10.2% and 10.6% in Kampala and Kamuli, respectively. Compared to males, females were more likely to be obese (2.9% vs. 1.8%) or overweight (17.4% vs. 3.3%). Residing in the city, alcohol consumption, smoking, non-engagement in sports activities, commuting to school by taxi or private vehicle and being from a rich family were the main factors significantly associated (P<0.05) with obesity. Being female (p = 0.0001) and not engaging in any sports activities (P = 0.002) were two factors significantly associated with being overweight. Conclusion: We observed significant gender differences in the prevalence of obesity among young adults in Uganda. Contrary to expectation, we did not observe significant rural-urban differences in the prevalence of overweight.