Summary Background Despite concern, few studies have been done about sexual violence against girls younger than 18 years of age in sub-Saharan Africa. We report the prevalence and circumstances of sexual violence in girls in Swaziland, and assess the negative health consequences. Methods We obtained data from a nationally representative sample of girls and women aged 13–24 years from selected households in Swaziland between May 15, 2007, and June 16, 2007, with a two-stage cluster design. The questionnaire examined demographics, type of sexual violence that took place before the respondent was 18 years of age, circumstances of the incident, and health-related conditions. Information was gathered from 1244 women and girls (response rate 96·3%), of whom 1242 provided retrospective responses to questions about sexual violence. We used regression models adjusted for relevant demographics to estimate the odds ratios for the associations between sexual violence and health-related conditions. Findings 33·2% (95% CI 29·9–36·7) of respondents reported an incident of sexual violence before they reached 18 years of age. The most common perpetrators of the first incident were men or boys from the respondent's neighbourhood (32·3% [28·8–36·1]) and boyfriends or husbands (26·2% [22·2–30·7]). The first incident most often took place in the respondent's own home (26·1% [21·6–31·2]). Sexual violence was associated with reported lifetime experience of sexually transmitted diseases (adjusted OR 3·69 [95% CI 1·78–7·66]), pregnancy complications or miscarriages (3·54 [1·47–8·55]), unwanted pregnancy (2·92 [1·87–4·55]), and self-report of feeling depressed (2·30 [1·70–3·11]). Interpretation Knowledge of the high prevalence of sexual violence against girls in Swaziland and its associated serious health-related conditions and behaviours should be used to develop effective prevention strategies. Funding UNICEF.