The purpose was to assess the validity of four generations of Xiaomi Mi Band wristbands for the assessment of step count and physical activity (PA) levels among adolescents aged 12-18 years under free-living conditions. One hundred adolescents were invited to participate in the present study. The final sample consisted of 62 high-school students (34 females), aged 12-18 years old (M_age = 14.1 ± 1.6 years), who wore an ActiGraph accelerometer on their hip (PA and step count reference measures) and four activity wristbands (Xiaomi Mi Band 2, 3, 4, and 5) on their non-dominant wrist during the waking time of one day. Results showed that the agreement between daily PA levels (i.e., slow, brisk, and slow-brisk pace walking, total PA and moderate-to-vigorous PA) measured by Xiaomi Mi Band wristbands and the accelerometer were poor (ICC, 95% CI = 0.06-0.78, 0.00-0.92; MAPE = 50.1-150.6%). However, agreement between daily step count measured by the accelerometer and the Xiaomi Mi Band wristbands were between acceptable (MAPE = 12.2-13.6%) to excellent (ICC, 95% CI = 0.94-0.95, 0.90-0.97). Furthermore, the Xiaomi Mi Band wristbands have a good to excellent validity for correctly classifying adolescents as meeting or not meeting the recommended 10,000 steps per day (P = 0.89-0.95, k = 0.71-0.87) and the recommended 60 minutes of moderate-to-vigorous PA per day (P = 0.89-0.94, k = 0.69-0.83). Furthermore, comparability between the four Xiaomi Mi Band generations were poor to excellent (ICC, 95% CI = 0.22-0.99, 0.00-1.00) for the daily PA levels outputs, although it was excellent (ICC, 95% CI = 0.99-1.00, 0.96-1.00; MAPE = 0.0-0.1%) for daily step count. Different models of Xiaomi Mi Band wristbands were comparable and presented good validity for measuring adolescents’ step count, and they accurately classified adolescents as meeting or not meeting the PA recommendations under free-living conditions.

• The Xiaomi Mi Band 2, 3, 4, and 5 wristbands were valid for measuring adolescents’ step count under unstructured free-living conditions.

• The Xiaomi Mi Band 2, 3, 4, and 5 wristbands accurately classified adolescents as meeting or not meeting the recommendations of 10,000 steps and the recommendation of 60 minutes of moderate-to-vigorous physical activity per day (based on the brisk pace walking score) under unstructured free-living conditions.

• The different generations of Xiaomi Mi Band wristbands achieve comparable results for measuring adolescents’ step count, time involved in brisk pace walking, slow-brisk pace walking, and total physical activity; however, results were poor for measuring time involved in moderate-to-vigorous physical activity.

• Options offered by the different generations could be an important reason to select one or another for a particular research context, as well as, they could be mixed during intervention programs.