Context: People with multimorbidity (?2 chronic pathologies within an individual) are more prone to experience mental health alterations and deleterious health behaviours. Therefore, they are more vulnerable to mental and behavioural alterations during COVID-19 confinement. Objective: Analyze variations in mental health and health behaviours during confinement between people with and without multimorbidity. Methods: Longitudinal cohort study consisting of two online questionnaires (1st confinement (05/2020) and post-deconfinement (11/2020)). Including 559 (phase 1) and 144 (phase 2) participants of average age 34.30±12.35 years old. Most of the participants were females (80.44%; 82.64%) living in Canada, France, India and Lebanon. Results: The prevalence of multimorbidity was 27.68%. People with multimorbidity were more likely to experience psychological distress (OR 2.48, 95%CI 1.57-3.92, p<0.001), depressive symptoms (OR 3.09, 95%CI 2.07-4.64, p<0.001), stress (OR 2.33, 95%CI 1.58-3.44, p<0.001) or feelings of isolation (OR 1.92, 95%CI 1.21-3.01, p=0.004) during confinement. Analysis showed decreases in physical activity (-74,41, SE=27,65, p=0.007) and increases in sleep problems (-0.15, SE=0.06, p=0.01) and fruit and vegetable consumption (4.38, SE=1.8, p=0.01) within people with multimorbidity during confinement, being not significant post-deconfinement. Conclusion: Mental and behavioural alterations occurred during the COVID-19 confinement. People with multimorbidity were affected differently by confinement emphasising the necessity for customised confinement strategies for this population.