Abstract: Introduction: Anxiety is one of the most common psychological problems in children, with an increasing trend during the COVID-19 pandemic. This study examines the mediating role of intolerance of uncertainty and cognitive emotion-regulation strategies in the relationship between stress and anxiety symptoms of children and early adolescents during the COVID-19 pandemic. Methods: We conducted the study with 234 elementary school students (128 females) aged 11 to 15 years. The students completed the COVID-19 Stress Scale, Revised Child Anxiety and Depression Scale, Intolerance of Uncertainty Scale for Children, and the Cognitive Emotion Regulation Questionnaire. To determine whether stress affects anxiety symptoms both directly and indirectly through intolerance of uncertainty and cognitive emotion-regulation strategies, we performed a mediation analysis with gender as a covariate. Results: Our results confirmed cognitive emotion regulation and intolerance of uncertainty as important cognitive vulnerability factors for anxiety in children and adolescents. A stressful experience contributes directly to anxiety and indirectly through higher intolerance of uncertainty and less adaptive strategies. The results serve to further improve prevention and intervention programs for children and adolescents which focus on correcting maladaptive cognitive emotion-regulation strategies and increasing tolerance of uncertainty.