We explored the prevalence and predictors of transactional sex with casual partners and main girlfriends among 1,288 men aged 15-26 from 70 villages in the rural Eastern Cape province of South Africa. Data were collected through face-to-face interviews with young men enrolling in the Stepping Stones HIV prevention trial. A total of 17.7% of participants reported giving material resources or money to casual sex partners and 6.6% received resources from a casual partner. Transactionally motivated relationships with main girlfriends were more balanced between giving (14.9%) and getting (14.3%). We constructed multivariable models to identify the predictors for giving and for getting material resources in casual and in main relationships. Each model resulted in remarkably similar predictors. All four types of exchange were associated with higher socio-economic status, more adverse childhood experiences, more lifetime sexual partners, and alcohol use. Men who were more resistant to peer pressure to have sex were less likely to report transactional sex with casual partners, and men who reported more equitable gender attitudes were less likely to report main partnerships underpinned by exchange. The most consistent predictor of all four types of transaction was the perpetration of intimate partner violence and rape against women other than a main partner. The strong and consistent association between perpetration of gender-based violence and both giving and getting material goods from female partners suggests that transactional sex in both main and casual relationships can be viewed within a broader continuum of men's exercise of gendered power and control. HIV prevention interventions need to explicitly address transactional sex in the context of ideas about masculinity which place a high emphasis on heterosexual success with, and control of, women.