Use of weight-related self-monitoring (WRSM) apps is common among emerging adults, as are weight and shape concerns. The present study aimed to examine (1) whether emerging adult use of dietary-focused (e.g., MyFitnessPal) and physical activity-focused (e.g., Fitbit) WRSM apps was associated with weight-control and muscle-building behaviors, including commonly recommended/conventional behaviors and disordered behaviors and (2) whether prior use of weight-control and muscle-building behaviors in adolescence might explain such relationships. Data were collected as part of the EAT (Eating and Activity over Time) 2010-2018 study (n = 1446) and analyzed using gender-stratified logistic regression models adjusted for demographic characteristics and body mass index. Among women and men, physical activity- and dietary-focused app use were associated with greater adjusted prevalence of disordered weight-control behaviors (e.g., fasting, purging), and disordered muscle-building behaviors (e.g., using steroids). Physical activity- and dietary-focused app use were also associated with a higher adjusted prevalence of commonly recommended weight-control and conventional muscle-building behaviors (e.g., exercising, changing eating habits), but only among those who were also engaging in disordered behaviors. The observed associations remained statistically significant in models that further adjusted for adolescent use of the respective behaviors. Findings suggest that emerging adults who use physical activity- and dietary-focused WRSM apps are more likely to engage in disordered weight-control and muscle-building behaviors and that associations are not explained by engagement in these behaviors during adolescence. Future research is needed to examine if there are aspects of WRSM apps that could be modified to reduce potential harm.