Sexual agreements are ubiquitous among male couples, yet little is known about motivations behind agreements and their association with sexual risk for human immunodeficiency virus (HIV). Qualitative interviews with 39 couples informed the development of the items in the Motivations Behind Agreement (MBA) scale. The scale was validated via exploratory factor analysis (EFA) and confirmatory factor analysis (CFA) using quantitative data from two (790 and 882 men) independent samples of couples. The EFA yielded two factors: relationship quality enhancement motive (RQEM) and sex life enhancement motive (SLEM). The CFA indicated satisfactory global model data fit. Finally, in predicting sexual risk, RQEM and SLEM showed significant interactions with agreement type. For men with monogamous agreements, higher RQEM was associated with less condomless anal sex (CAS) with an outside partner of discordant or unknown serostatus (aOR = 0.15; 95% CI = 0.05, 0.46). For men with nonmonogamous agreements, higher RQEM was associated with less CAS (aOR = 0.76; 95% CI = 0.60, 0.97) while higher SLEM was associated with greater odds of CAS (aOR = 1.57; 95% CI = 1.18, 2.08). Men whose agreements were highly motivated by relationship enhancement were less likely to engage in sexual risk with outside partners regardless of agreement type. HIV-prevention interventions targeting male couples will benefit from incorporating an understanding of couples' agreement motivations.