While there are a number of short personality trait measures that have been validated for use with adults, few are specifically validated for use with adolescents. To trust such measures, it must be demonstrated that they have adequate construct validity. According to the view of construct validity as a unifying form of validity requiring the integration of different complementary sources of information, this article reports the evaluation of content, factor, convergent, and criterion validities as well as reliability of adolescents’ self-reported personality traits. Moreover, this study sought to address an inherent potential limitation of short personality trait measures, namely their limited conceptual breadth. In this study, starting with items from a known measure, after the language-level was adjusted for use with adolescents, items tapping fundamental primary traits were added to determine the impact of added conceptual breadth on the psychometric properties of the scales. The resulting new measure was named the Big Five Personality Trait Short Questionnaire (BFPTSQ). A group of expert judges considered the items to have adequate content validity. Using data from a community sample of early adolescents, the results confirmed the factor validity of the Big Five structure in adolescence as well as its measurement invariance across genders. More important, the added items did improve the convergent and criterion validities of the scales, but did not negatively affect their reliability. This study supports the construct validity of adolescents’ self-reported personality traits and points to the importance of conceptual breadth in short personality measures.