Background: Women in Sub-Saharan Africa are disproportionately affected by the HIV epidemic. Young women are twice as likely to be living with HIV than men of the same age and account for 64% of new HIV infections among young people. Many studies suggest that financial needs, alongside biological susceptibility, are the main causes of the gender disparity in HIV acquisition. While the literature shows a limited understanding of the link between poverty and HIV, there is some new robust evidence demonstrating that women adopt risky sexual behaviours as a way to cope with economic shocks. Methods: We recruited 1,506 adolescent girls and women engaging in transactional or in commercial sex in Yaounde, Cameroon, using snowball sampling. Half of the study participants were randomly allocated to receive a free health insurance product covering themselves and their economic dependents over 12 months. We collected data on socio-economics, health and sexual behaviours and sexually transmitted infection and HIV biomarkers at baseline, 6-month post randomisation (midline) and 12-month post randomisation (endline). Results: We found that study participants engaging in transactional sex allocated to the treatment group were less likely to be infected with HIV (OR=0.109, p-value<0.05). We showed that the intervention allowed women to leave transactional sex. In addition, we found that for the participants remaining in transactional sex, the intervention increased condom use and reduced sex acts, these results were however only statistically significant at 10% given our sample size. There was no evidence of a change in risky sexual behaviours or in a reduction of HIV incidence among female sex workers. Conclusion: The study provides the first evidence of the effectiveness of a formal shock-coping strategy to prevent HIV among women in Africa. We showed that the intervention effectiveness operates through the reduction in health shocks since the increase in healthcare use following the intervention was low. We estimated that in our trial, the cost for each HIV infection averted is £4,667 among the cohort of women engaging in transactional sex. Policymakers should consider formal shock-coping strategies to prevent HIV among women in Africa.