This study aim to assess changes in obesity and activity patterns among youths in China during the COVID‐19 lockdown. We used the COVID‐19 Impact on lifestyle change survey (COINLICS), a national retrospective survey distributed via social media platforms in early May 2020 where more than 10 000 youth participants in China have voluntarily reported their basic sociodemographic information, weight status, and routine lifestyles in the months before and during COVID‐19 lockdown. The extended IOTF and WHO standards were used to define overweight and obesity of the participants. We used paired t‐tests or χ 2 tests and non‐parametric methods to evaluate the significance of differences in weight‐related outcomes and lifestyles across education levels, between sexes, and before and during COVID‐19 lockdown. The mean body mass index of all participating youths has significantly increased (21.8‐22.6) and in all education subgroups during COVID‐19 lockdown. Increases also occurred in the prevalence of overweight/obesity (21.3%‐25.1%, P < .001) and obesity (10.5% to 12.9%, P < .001) in overall youths, especially in high school and undergraduate students. Their activity patterns had also significantly changed, including the decreased frequency of engaging in active transport, moderate−/vigorous‐intensity housework, leisure‐time moderate−/vigorous‐intensity physical activity, and leisure‐time walking, and the increased sedentary, sleeping, and screen time. Our findings would inform policy‐makers and clinical practitioners of these changes in time, for better policy making and clinical practice. School administrators should also be informed of these changes, so in‐class and/or extracurricular physical activity programs could be designed to counteract them.