The Penn State Worry Questionnaire for Children (PSWQ-C) is a commonly used instrument for evaluating worry in children. This study investigated the measurement invariance of the Polish version of the PSWQ-C through two types of research: paper-pencil and online. The results replicate the good psychometric properties of the PSWQ-C. The bifactorial model, with one general factor feature and two method factors related to the way the scale statements are formulated (positive and negative), is the model that best fits the data. Multigroup confirmatory factor analysis (MCFA) supports measurement invariance (configural, metric, scalar) across paper-pencil and online research.