The purpose of the study was to estimate the prevalence of insufficient physical activityand associated factors in adolescents 10 to 19 years old living in the Health Districts of Restinga and Extremo Sul of Porto Alegre, RS. A population-based, cross-sectional study was conducted by a systematic sampling with 967 adolescents randomly selected. Data was collected by using a questionnaire withthe following variables: gender, age, skin color, if currently studying, occupation, socioeconomic status, weight and height, as well as insufficient physical activity, which was defined as less than 300 minutes per week of physical activity, not counting physical education classes. The prevalence of insuficiente physical activity found was 70.5%: 58.9% in boys and 81.9% in girls. The higher prevalence of insufficient levels of PA was found among adolescents who are not studying (p=0.01) and next by those who are between 16 and 19 years old (p=0.05). In boys it was observed that as they got older, physical activity decreased (p=0.05). The most types of activities found were field soccer and riding bikes for boys, and walking and field soccer for girls. In conclusion,the prevalence of insufficient physical activity found in the adolescentes evaluated was high. Being a girl, not studying, and between the ages of 16 and 19, were factors that increased the levels of insufficient physical activity. This reveals, therefore, the importance to developed interventions to promote the practice of physical activityin this population.