Physical activity (PA) levels among adolescents are low globally and in Vietnam. This study aims to assess the prevalence of PA and identify demographic, psychosocial, and behavioral factors related to PA among secondary school students in Ho Chi Minh City. A cross-sectional study was conducted from March to May 2023, involving 475 6th-grade students. Data were collected through self-reported questionnaires and anthropometric measurements. Psychosocial factors, including self-efficacy, enjoyment, family influence, and belief in PA were measured using validated scales, while teacher influence on PA was evaluated using a custom-designed questionnaire. Sedentary behavior was assessed using the Adolescent Sedentary Activity Questionnaire (ASAQ), and PA levels were determined using the Physical Activity Questionnaire for Older Children (PAQ-C). Gender differences were analyzed using chi-square tests and t-tests. Pearson's correlation coefficient and hierarchical multiple regression were applied to examine associations and factors influencing PA. Only 36.8% of students met the recommended PA levels, with boys (40.7%) being more active than girls (32.9%). Boys had higher body mass index (BMI) and more screen time, while girls spent more time on cultural and social activities. Positive correlations were found between PA levels and psychosocial factors, especially in self-efficacy (r = 0.761, p < 0.001) and family influence (r = 0.717, p < 0.001). Sedentary behaviors, such as small screen recreation (r = -0.449, p < 0.001) and after-school education (r = -0.380, p < 0.001), negatively correlated with PA levels. Hierarchical analyses showed that BMI, self-efficacy, beliefs, family and teacher influence, and time spent on small screen recreation and after-school education significantly explained PA among adolescents. Vietnamese adolescents have low PA levels, with gender differences. Interventions should enhance support, reduce sedentary behavior, and address gender-specific barriers.