Background: Physical activity (PA) assessment by questionnaire is a cornerstone in the field of sport epidemiology studies. The Physical Activity Questionnaire for Children (PAQ-C) has been used widely to assess PA in healthy school populations. The aim of this study was to evaluate the reliability and validity of the PAQ-C questionnaire in Spanish children using triaxial accelerometry as criterion. Methods: Eighty-three (N = 46 boys, N = 37 girls) healthy children (age 10.98 ± 1.17 years, body mass index 19.48 ± 3.51 kg/m[superscript 2]) were volunteers and completed the PAQ-C twice and wore an accelerometer for 8 consecutive days. Reliability was analyzed by the intraclass correlation coefficient (ICC) and the internal consistency by the Cronbach's a coefficient. The PAQ-C was compared against total PA and moderate to vigorous PA (MVPA) obtained by accelerometry. Results: Test-retest reliability showed an ICC = 0.96 for the final score of PAQ-C. Small differences between first and second questionnaire administration were detected. Few and low correlations (rho = 0.228-0.278, all ps < 0.05) were observed between PAQ-C and accelerometry. The highest correlation was observed for item 9 (rho = 0.311, p < 0.01). Conclusions: PAQ-C had a high reliability but a questionable validity for assessing total PA and MVPA in Spanish children. Therefore, PA measurement in children should not be limited only to self-report measurements.