Physical activity is an important component of a healthy lifestyle. Evaluating and promoting this activity warrants careful attention, particularly in the context of preventing obesity associated with low physical activity. The aim of the present study was to assess the concurrent validity of a modified, long, self-administered version of the International Physical Activity Questionnaire (IPAQ) using triaxial accelerometry in 56 youth with obesity. The participants' physical activity was measured with an RT3 accelerometer (objective measure) and with the International Physical Activity Questionnaire for Adolescents (IPAQ-A; subjective measure). Although the two methods were correlated strongly (r = .84), there was a significant difference between the two methods. The participants' subjective ratings on the IPAQ-A overestimated the objective measure. Although activity questionnaires are simple and widely used in assessing physical activity, alternative methods should be used when measuring physical activity in youth with obesity.