Risky sexual behaviour continues to be the main means through which sexually transmitted infections (STIs) and HIV are spread in South Africa. However few studies have assessed risky sexual behaviour among married and cohabiting women in this high HIV prevalence country. The aim of this study was therefore to assess the prevalence and identify the predictors of risky sexual behaviour among married and cohabiting women in Mahikeng Local Municipality where HIV is generalized. The study used cross-sectional mixed methods data obtained from 568 women responding to a survey and 33 women participating in in-depth interviews in 2012. All the women were aged 18-49 years, married or cohabiting and were residing in rural and urban areas of Mahikeng Local Municipality at the time of the study. The data was analyzed by the use of the Chi square statistic and a parsimonious binary logistic regression model, and the thematic content analysis method. The study found that 4 in 10 of the women experienced risky sexual behaviour, which was significantly predicted by being in marriage or cohabitation for \5 years, having a partner working in agriculture, government and mining sectors, having sex to achieve motherhood, having the perception by the women that they were unlikely to contract STIs, having the belief that partners have the right to use force to obtain sex and having partners who were unwilling to use condoms. All the above predictors were exacerbated by culturally sanctioned gender inequality and structural factors. The findings suggest that risky sexual behaviour is prevalent among married and cohabiting women in the study area which has serious implication for the marital transmission of STIs and HIV. We suggest interventions that enhance gender equality in sexual decisions and initiatives that encourage men to change their sexual beliefs and values.