Objectives: This study is determining the effect of the COVID-19 lockdown on dietary habits and physical activity among children and adolescents living in the Kingdom of Saudi Arabia (KSA). Methods: Cross-sectional descriptive data were collected from April–June 2020 through clinical interview questions via telecommunication at a virtual pediatric endocrine clinic. Results: More than half (58.4%) had appetite changes. Moreover, 39.4% had difficulty maintaining a healthy, balanced diet during the pandemic, including an increase in the consumption of simple carbohydrates, fried foods, and soft drinks. Those with a higher socio-economic status had increased snack food consumption (p<0.047). There was a significant decrease in physical activity among 58.2% of the children/adolescents, 42% did not exercise, and 28.2% exercised <30 minutes per day. Over one-third (35%) had a significant increase (4–8 hours) in screen time. Girls reported more changes to their daily chores than did boys (p<0.036). Online learning affected boys’ activity levels more so than girls’ (p<0.011). Conclusion: This study confirmed the negative impact of COVID-19 lockdown on weight, dietary habits among children/adolescents living in the KSA, highlighting possible adverse health outcomes in the future.