Background: Loneliness, a feeling of distress, has aggravated due to the COVID-19 pandemic lockdowns and reduced social interactions. The objective of this study was to explore whether increased loneliness due to the COVID-19 pandemic was associated with various eating and activity behaviours in adolescence, a critical period for the development of lasting lifestyle habits. Methods: In this cross-sectional study, we used self-reported data from 43,588 and 40,521 Canadian adolescents aged 12-19 years (collected between November 2020 and June 2021) for eating habits and the 24-hour movement behaviours, respectively. Binary and multinomial logistic regression were used to predict the odds of various lifestyle behaviours among adolescents with increased loneliness due to the COVID-19 pandemic. Results: We found higher odds of skipping breakfast [boys: OR 1.41 (95% CI: 1.33, 1.50), girls: OR 1.64 (95% CI: 1.56, 1.74)], fast food consumption [1-2 days in the past week: girls - OR: 1.14 (95% CI: 1.08, 1.21); ≥3 days in the past week: boys - 1.12 (95% CI: 1.02, 1.24), girls - OR: 1.42 (95% CI: 1.29, 1.57)], not meeting screen time [boys: OR 1.43 (95% CI: 1.24, 1.66), girls: OR 1.72 (95% CI: 1.54, 1.92)], and sleep duration guidelines [boys: OR 1.38 (95% CI: 1.28, 1.48), girls: OR 1.36 (95% CI: 1.27, 1.45)] among adolescents that reported increased loneliness due to the pandemic (versus those in the decreased/stayed the same loneliness group). Conclusion: Future longitudinal studies in adolescents are needed to confirm the directionality of these associations. It is important to raise awareness of these findings among public health practitioners, policymakers, physicians, schools and parents to promote healthier eating habits and increase adherence to the 24-hour movement behaviours. Recovery efforts post-pandemic are needed to reduce loneliness levels to support adolescent social health and establish healthy behavioural habits across the lifespan.