While sexual partner age disparity is frequently considered as a potential risk factor for HIV among young women in Africa, no research has addressed this question among older women. Our aim was thus to determine whether sex partner age disparity was associated with subsequent HIV acquisition in women over 30 years of age. To achieve this aim we conducted a quantitative analysis of a population-based, open cohort of women in rural KwaZulu-Natal, South Africa (n = 1,737) using Cox proportional hazards models. As partner age rose, HIV acquisition risk fell significantly: compared to a same-aged partner, a 5-year older partner was associated with a one-third reduction [hazard ratio (HR) 0.63, 95 % CI 0.52-0.76] and a 10-year older partner with a one-half reduction (HR 0.48, 95 % CI 0.35-0.67) in acquisition risk. This result was neither confounded nor effect-modified by women's age or socio-demographic factors. These findings suggest that existing HIV risk-reduction campaigns warning young women about partnering with older men may be inappropriate for older women. HIV prevention strategies interventions specifically tailored to older women are needed.