Abstract Aim: The purpose of this study was to analyse the effects of age, sex, body mass index (BMI) and country of origin (Mozambique versus Portugal) on physical activity and sedentariness. Methods: Sample size was comprised of 593 adolescents from Mozambique (n=258) and Portugal (n=335), aged 10-15 years old. Height and weight were measured, and BMI was computed; furthermore, subjects were classified in two groups (normal weight and overweight/obese). The mean time (min/day) spent in moderate-to-vigorous physical activity (MVPA), light physical activity (LPA), and sedentary activity were assessed using the GT3X Actigraph accelerometer over at least 4 days (1 weekend day) with a minimum of 10 hours/day of monitoring. Multiple regression analysis was done in SPSS 20. Results: A low percentage of Mozambican (3.9%) and Portuguese (4.3%) youth complied with the MVPA recommendations (60min·day-1). However, Portuguese boys spend more time in MVPA. Girls and those with higher BMI spend more time in sedentary activities and less time in MVPA and LPA; older adolescents spend more time in sedentary activities and less time in LPA. There is a significant effect of the country of origin on MVPA, favouring Portuguese youth. Conclusions: Results suggest country differences in MVPA between Mozambican and Portuguese youth - the Mozambicans are less involved in MVPA than the Portuguese, probably reinforcing the undesirable effect of the urbanization process in developing nations.