Background <p>Various interventions have been developed with the intention to increase the amount of physical activity among children, including those promoting active school travel (AST). However, no gold standard currently exists for measuring different travel modes in AST. This study evaluates the criterion validity of a web-based data collection tool designed for children to self-report their school travel.</p> Purpose <p>To assess the criterion validity of a web-based data collection tool for daily self-registration of commuting mode, time, and distance among middle school children in Sweden.</p> Methods <p>Thirty children (10–12 years old) from six schools in Falun, Sweden, were recruited using snowball sampling. The children self-reported their school travel data for one day, including travel mode, commuting time, and distance. Their reports were compared to a criterion standard based on direct observations. Spearman correlation and the Wilcoxon signed-rank test were used to analyze the accuracy of the self-reported data.</p> Result <p>The web-based data collection tool demonstrated 100% agreement between self-reported and observed travel modes, with high correlations for commuting time and distance (r_s=0.953–0.989, <i>p</i> &lt; 0.001). The Wilcoxon signed rank test showed no significant differences between self-reported data and criterion standard (<i>p</i> = 0.243–0.903).</p> Conclusion <p>The strong agreement between self-reported and observed travel data indicates high criterion validity, suggesting that the web-based data collection tool is a reliable method for middle school children to self-report their daily school travel.</p>