Transactional sex has been associated with risk of HIV infection in a number of studies throughout sub-Saharan Africa. Urban young women are economically vulnerable and at heightened risk of HIV infection in Tanzania; yet there are few studies that have explored relationship dynamics, including transactional sex, in this setting. This paper sheds light on the broader context of sexual relationships among youth at risk for HIV, how transactional sex plays out in these relationships, and how the transactional nature of relationships affects women’s risk for HIV. We conducted 60 in-depth interviews and 14 focus group discussions with young men and women, 16-24 years old, in Dar es Salaam, Tanzania. These data guided the development of a community-based HIV and violence prevention intervention for young men. Youth described the exchange of sex for money or other material goods in all types of sexual relationships. While the exchange was explicit in casual relationships, young women voiced material and monetary expectations from their committed partners as well. Young men described their pursuit of multiple partners as sexually motivated, while women sought multiple partners for economic reasons. Young men were aware of the expectations of material support from partners, and acknowledged that their ability to provide for a partner affected both the longevity and exclusivity of their relationships. Youth described a deep mistrust of the motivations and commitment of their sexual partners. Furthermore, young women’s financial dependence on men impacted their ability to negotiate safe sexual behaviors in both casual and committed relationships. Programs designed to reduce HIV risk among Tanzanian youth need to take into account the transactional component of sexual relationships and how such exchanges differ according to partner type.