Background Health promotion of children and adolescents through participation in purposeful and meaningful activities is one of the focuses of occupational therapy. In this study, in line with the Iranian culture, a tool for assessment of Iranian adolescents’ participation in daily occupations/activities was developed and validated. Method An exploratory sequential mixed method was used to develop and validate the tool. Content and structural validity and reliability of the initial version were estimated. Results During the validation process, the items were reduced to 54. Data were collected from 481 adolescents to investigate factor analysis. According to principal component analysis, the Kaiser–Meyer–Olkin index was 0.88, and the analysis yielded seven factors explaining 47.2% of the total variance. The tool showed excellent internal consistency and test–retest reliability. Conclusion This newly developed Participation Assessment Tool for Iranian adolescents is a valid and reliable measure for assessing the participation of this group in daily activities.