The pubertal transition is characterized by pronounced sex hormone fluctuation, refinement of affective neural circuitry, and an increased risk of depression in female adolescents. Sex hormones, including testosterone, exert modulatory effects on frontal-limbic brain networks and are associated with emotion dysregulation and depressive symptoms. Weekly changes in hormones predict affective symptoms in peripubertal female adolescents, particularly in the context of stress; however, the biobehavioral mechanisms underlying hormone change and mood relationships during the pubertal transition have yet to be determined and was the objective of the present study. Forty-three peripubertal female adolescents (ages 11-14) collected 8-weekly salivary hormone (estrone, testosterone) samples and mood assessments to evaluate hormone-mood relationships, followed by a biobehavioral testing session with psychosocial stress and EEG. Within-person correlations between weekly hormone changes and corresponding mood were performed to determine individual differences in mood sensitivity to weekly hormone change. Increased frontal theta activity indexing emotion reactivity, reduced cortisol reactivity, and reduced vagal efficiency predicted the strength of the relationship between testosterone and mood. Further, testosterone-sensitivity strength was associated with the enhancement of negative affect following stress testing. Results identify divergent frontal theta and stress responses as potential biobehavioral mechanisms underlying mood sensitivity to peripubertal testosterone fluctuation.