Background In recent decades, changes in food consumption among adolescents in Arabic speaking countries, including The Kingdom of Saudi Arabia (KSA), has resulted in increased burden of overweight and obesity. COVID-19 related restrictions have further impacted lifestyle and diet among adolescents in KSA. Aim The present study aimed to assess the knowledge, attitude, behaviour and dietary patterns of adolescents and the impact of the COVID-19 pandemic related restrictions on these behaviours. Methods A cross-sectional study was conducted in six intermediate schools among 1500 adolescents (11-15 years) via an anonymous online survey. The Chi Square test was used to compare gender stratified nutritional knowledge, attitude and behaviour of adolescents and the impact of COVID-19 related restrictions. To determine the factors associated with nutritional change during COVID-19 lockdown, logistic regression was employed using the categorisation of nutritional change during COVID-19 restrictions versus no change in univariate and multivariate analyses. Results A good level of nutrition-related knowledge and attitude was demonstrated both by female and male students, with 66% and 57% of participants scoring above the median of the total possible knowledge and attitude score, respectively. After adjusting for all variables, higher nutrition knowledge score predicted students to be less likely to report increased food consumption during COVID-19 restrictions, whereas students with higher nutrition attitude scores and with a higher risk of increased food consumption were more likely to increase food consumption during COVID-19 restrictions. Conclusions Unhealthy dietary habits among Saudi adolescents are of paramount public health concern and the COVID-19 restriction period exacerbated the pre-pandemic nutrition-related unhealthy behaviours. Therefore, school-based interventions aiming to promote healthy lifestyles and the wellbeing of Saudi adolescents and use of qualitative recall dietary diary is also crucial to develop programs that prevent unhealthy lifestyles in adolescents in Saudi Arabia.