Individual differences in salivary testosterone were examined in 213 adolescents (106 boys, 107 girls; mean age = 13.66 years) in relation to externalizing and internalizing psychopathology. Self- and parent-report measures of behavior problems and psychiatric symptoms were obtained. Latent anxiety–depression, disruptive behavior, and attention problem constructs were developed using multitrait, multimethod procedures. Saliva samples were collected in the morning, midday, and late afternoon on multiple days and were later assayed for testosterone. Latent constructs were derived for testosterone level and diurnal variation across the six sampling occasions. Structural equations modeled relationships between problem behavior and intra- and interindividual differences in testosterone separately by gender. For boys, lower levels of testosterone and testosterone levels that decreased more slowly across the day were related to higher levels of anxiety–depression and attention problems. These associations were not moderated by pubertal development. For girls, steep declines in testosterone production across the day related to higher levels of disruptive behavior problems, but this association was only evident after including pubertal development as a moderator in the model. These findings raise novel questions regarding the nature and magnitude of links between testosterone and problem behavior in youth.