Objective: Early pubertal timing has consistently been associated with internalizing psychopathology in adolescent girls. Here, we aimed to examine whether the association between timing and mental health outcomes varies by measurement of pubertal timing and internalizing psychopathology, differs between adrenarcheal and gonadarcheal processes, and is stronger concurrently or prospectively. Methods: We assessed 174 female adolescents (age 10.0-13.0 at Time 1) twice, with an 18-month interval. Participants provided self-reported assessments of depression/anxiety symptoms and pubertal development, subjective pubertal timing, and date of menarche. Their parents/guardians also reported on the adolescent’s pubertal development and subjective pubertal timing. We assessed salivary DHEA, testosterone and estradiol levels, and conducted clinical interviews to determine the presence of case level (DSM-IV and HiTOP) internalizing disorders. From these data, we computed 12 measures of pubertal timing at both time points, as well as 7 measures of internalizing psychopathology, and entered these in a Specification Curve Analysis. Results and Conclusion: Overall, earlier pubertal timing was associated with increased internalizing psychopathology cross-sectionally and prospectively. However, results varied by measure of pubertal timing and psychopathology, with the strongest associations when pubertal timing was based on the Tanner Stage Line Drawings and when the outcome was case-level DSM-IV depression or HiTOP distress disorders. Timing based on hormone levels was not associated with internalizing psychopathology, suggesting that psychosocial mechanisms, captured by timing measures of visible physical characteristics, are more meaningful determinants of internalizing psychopathology than biological ones in early adolescent girls. Future research should precisely measure and test these psychosocial mechanisms.