Stable serodiscordant relationships and sexual concurrency are pathways that contribute to the HIV epidemic in sub-Saharan Africa. However whether polygyny imparts the same risks as informal concurrent relationships remains an open research question. Using data collected at enrollment from a cohort study of sero-discordant couples, this analysis investigates how polygynous relationships differ from those involving only a single female spouse and whether men involved in polygynous partnerships are more likely to report HIV-risk behaviour than those in single spouse partnerships. Of 444 enrolled couples, 111 (25%) were polygynous and 333 (75%) were single-spouse partnerships. We found that polygynous men were more likely to report controlling sexual decision-making and to report any unprotected sex with unknown sero-status partner. After controlling for potential confounders, polygynous men were still more likely to report unprotected sex with an unknown sero-status partner. In this sample of sero-discordant couples we found indication of excess HIV-risk behaviour among men involved in polygynous relationships.