Understanding the social and cultural contextual determinants of sexual behaviour of adolescents and young adults is an essential step towards curtailing the spread of HIV. This study examined the effects of one cultural factor, ethnicity, on sexual abstinence, faithfulness, condom use at last sex, and risky sex among young people in Zimbabwe. Data from the cross-sectional, population-based 2005–06 Zimbabwe Demographic and Health Survey were used. Net of the effect of sociodemographic and social–cognitive factors, and using multinomial logistic regression, ethnicity was found to have a strong and consistent effect on sexual behaviour among youth. In addition, the study found that there were ethnic-specific and within-gender differences in sexual behaviour, for both men and women. Shona youth were more likely to be abstinent than Ndebele youth. Compared with Shona youth, Ndebele youth were more likely to have engaged in risky sex. However, Ndebele men were more likely have used condoms at last sex, compared with Shona men. For both men and women, sexual behaviour was more socially controlled. School attendance and religion exerted protective effects on sexual abstinence. For men only, those living in rural areas were less likely to be faithful and more likely to have engaged in risky sexual behaviour than those living in urban areas. The study attests to the fact that ethnic norms and ideologies of sexuality need to be identified and more thoroughly understood. In addition, the study provides evidence that in order to promote safe and healthy sexuality among young people in Zimbabwe, cultural, social and gender-specific approaches to the development of HIV prevention strategies should be seriously considered. Current success in the Abstinence, Being faithful and Condom use (ABC) approach could be strengthened by recognizing and responding to cultural forces that reproduce and perpetuate risky sexual behaviours.