Gender norms affect HIV risk within serodifferent partnerships. We assessed how the sexual relationship power described by men living with HIV (MLWH) associates with periconception HIV-transmission risk behavior. Quantitative surveys were conducted with 82 MLWH reporting a recent pregnancy with an HIV-negative or unknown-serostatus partner in KwaZulu-Natal, South Africa. Surveys assessed decision-making dominance (DMD) using the Pulerwitz et al. sexual relationship power scale; partnership characteristics; and HIV-risk behaviors. Multivariable logistic regression models evaluated associations between DMD score and HIV-risk behaviors. Higher male decision-making dominance was associated with non-disclosure of HIV-serostatus to pregnancy partner (aRR 2.00, 95% CI 1.52, 2.64), not knowing partner’s HIV-serostatus (aRR 1.64, 95% CI 1.27, 2.13), condomless sex since pregnancy (aRR 1.92, 95% CI 1.08, 3.43), and concurrent relationships (aRR 1.50, 95% CI 1.20, 1.88). Efforts to minimize periconception HIV-risk behavior must address gender norms and power inequities.