Despite population aging and the increase in mental health problems, studies on the mental health of older people in developing countries are lacking. This population-based cross-sectional study estimated the prevalence and associated factors for depression, anxiety and stress symptoms among older adults in Nepal. Community-dwelling older adults (N = 794) were interviewed using a questionnaire which consisted of the 21-item Depression Anxiety Stress Scales; and a wide range of sociodemographic, health-related, and lifestyle characteristics; functional ability, social support, participation in social activities, and adverse life events. The prevalence of symptoms was 15.4% for depression, 18.1% for anxiety, and 12.1% for stress. Risk factors for symptoms included female gender, working in agriculture, lower household wealth, perceived poor health, smoking, chronic conditions, migration of adult children, and exposure to adverse life events. Receiving an allowance, physical exercise, functional ability, social support, and participation in social activities were found to have protective effects. The findings indicate the need for community-based interventions, including appropriate diagnosis and treatment of mental health conditions, and mental health promotion programs targeting the risk and protective factors.