Positive affect and positive parent-adolescent relationships have been found to reinforce one another across youth development in a pattern of an "upward spiral," yet little is known regarding processes facilitating such "upward spirals" of social and emotional wellbeing among parent-adolescent dyads. This study addressed this gap by examining interpersonal capitalization, or the process of sharing positive news with others, as one candidate interpersonal process contributing to increases in both parent and adolescent experiences of positive affect in naturalistic settings. Participants included 146 adolescents (52.1% girls; ages 10-14; M[SD] = 12.71[0.86]) and a participating caregiver (N = 139; 78.7% mothers; ages 33-58; M[SD] = 44.11[5.08]) who completed a dyadic experience sampling method procedure assessing both parent and adolescent momentary affect and patterns of engagement in interpersonal capitalization in daily life settings (31 surveys across 9 days). Multilevel models indicated that adolescent positive affect increased following instances of interpersonal capitalization, and increases in positive affect were specific to high-arousal positive emotions (e.g., excited, energetic) relative to low-arousal positive emotions (e.g., calm, relaxed). Parental high-arousal positive affect also increased following instances in which they provided validating, enthusiastic responses to their children's capitalization attempts. The results of the present study indicate that interpersonal capitalization may be one mechanism facilitating "upward spirals" of positive affect for both parents and adolescents, with implications for health and wellbeing across development.