The purpose of this study was to examine the validity and the reliability of the Greek version of the self-report instrument “3day Physical Activity Record” (3dPAR-G, Pavlidou, Michalopoulou, Aggeloussis & Taxildaris, 2009). Participants were 57 high school students (Mean Age=14.02 y., SD=0.72 y.). Concurrent validity was assessed against the accelerometer (MTI/Actigraph) whereas reliability was assessed through two consecutive measurements that were performed two weeks apart. Participants wore the MTI/ Actigraph for five consecutive days and completed the 3dPAR-G for the last three of those days. Significant correlation was observed between MTI/Actigraph raw counts and 3dPAR-G average score (METs min. day-1, r = .41). Regarding reliability of the 3dPAR-G the intraclass correlation coefficient for total physical activity (MET/min. day-1), was significant (ICC = .57, p < .01). Finally, in terms of internal consistency, Cronbach’s alpha was estimated and was rather high (α = .81). Conclusively and within the limitations of the present study (small sample size, 5 days of objective physical activity monitoring), the results indicated that the 3dPAR-G is a valid and reliable instrument for assessing group-level reported physical activity in Greek adolescents aged 13-15 years.