Despite strong concerns that sexting poses risks for adolescents’ well-being, previous research finds mixed results. Moreover, these studies rely heavily on self-report measures and cross-sectional designs. This study utilizes observational methods to examine longitudinal relations between text-based sexting and both negative and positive indicators of psychosocial adjustment. An ethnically diverse sample of 197 adolescents was provided smartphones that captured their text messages across high school, from 2008–2012. Two, two-day samples of text messages from grades 9–12 were content-coded (468,201 total observations). Sexting was defined as sending and receiving text-based statements about past, present, or hypothetical sexual behaviors occurring between dyadic texting partners. Each year, adolescents reported on negative (internalizing, externalizing, and social problems; borderline personality features) and positive (life satisfaction, group belongingness, and positive self-perceptions) indicators of their psychosocial adjustment. Using concurrent and longitudinal multilevel models, greater sexting at one time point than one’s average (within-person) was only associated with lower levels of group belongingness within the same year. For girls only, text-based sexting more than others on average (between-person) was related to increased externalizing symptoms and borderline personality disorder features, as well as decreased life satisfaction, group belongingness, self-perceived social competence, and global self-worth. No within-person differences by gender emerged, nor did longitudinal associations. Future studies and intervention efforts should examine when and why higher sexting might be related to psychosocial problems among girls.