Transactional sex and sexual relationships with older partners increase HIV risk in adolescent girls and young women (AGYW), yet little is known about how these behaviors co-evolve over time. We characterize temporal patterns of transactional sex and age-disparate relationships among AGYW in South Africa. Longitudinal data are from a randomized controlled trial (HPTN 068) of school-aged, HIV-negative, AGYW who attended ≥ 3 study visits. We used group-based trajectory modeling to identify trajectories of transactional sex and age-disparate relationships (partner ≥ 5 years older) in the last year and assessed the interrelationship (conditional probability) between both trajectories. At baseline, median age was 14 years, 14.5% of girls were sexually active, and transactional sex (2.1%) and age-disparate relationships were uncommon (2.7%). We identified two trajectories for transactional sex ("low" [81.9%] and "increasing" [18.1%]) and two for age-disparate relationships ("low" [91.7%] and "increasing" [8.3%]). In a separate joint trajectory analysis, nearly a third (28%) had increasing trajectories for both transactional sex and age-disparate relationships, but most (53%) had a low trajectory of both outcomes. Baseline reporting of early sexual debut, depression, and inequitable gender norms were highest in the increasing transactional sex group. Prior pregnancy, early sexual debut, and IPV were highest among those with increasing age-disparate relationships. AGYW who engage in transactional sex or age-disparate partnerships in early adolescence are more likely to experience sustained engagement in both behaviors as they transition to adulthood, increasing HIV risk. Engaging girls early may maximize effectiveness of behavioral and biomedical HIV prevention efforts.