Background: South Africa has among the highest rates of intimate partner violence (IPV) globally, with young women at heightened risk due to inequitable gender roles, limited relationship skills, and inadequate social support. Despite an urgent need for low-cost, scaleable IPV solutions, most efficacious approaches are time-intensive and costly to deliver. Digital, interactive chatbots could deliver behaviourally-informed strategies to help young women navigate their relationships in the context of high IPV prevalence. Methods: Young women (18-24 years old) across South Africa were recruited via Facebook for participation in an individually randomised controlled trial (n=19,643). Users were randomly allocated, using a pipeline algorithm, to one of four trial arms: Pure Control (PC) had no user engagement outside of study measures; Attention Treatment (T0) provided didactic information about sexual health through a text-based chatbot; Gamified Treatment (T1) was a behaviourally-informed gamified text-based chatbot; Narrative Treatment (T2) was a behaviourally-informed drama delivered through pre-recorded voice notes. All chatbots were delivered in WhatsApp, through which users were invited to complete brief “quizzes” comprising adapted versions of validated scales. A direct chat link to a trained counsellor was a safety measure (and was accessed by 4·5% of the sample). Primary outcomes were adapted validated scales of gender beliefs and past-month IPV. Secondary outcomes were identification of unhealthy relationship behaviours and depressive symptoms. We estimated treatment effects using ordinary least squares and heteroskedasticity robust standard errors.Findings: Compared to control, all treatments were effective in improving gender beliefs (Cohen’s D=0·10, 0·29, 0·20 for T0, T1, and T2, respectively). The gamified chatbot (T1) had small but significant effects on IPV: 56% of young women reported past-month IPV, compared to 62% among those without treatment (marginal effects=-0·07, 95%CI=-0·09to-0·05). The narrative treatment (T2) had no effect on IPV exposure. T1 significantly increased identification of unhealthy relationship behaviours (Cohen’s D=0·25). Neither T1 nor T2 had a measurable effect on mental health, nor did either treatment arm cause mental distress.Interpretation: A behaviourally-informed, gamified chatbot increased gender equitable beliefs and skills and was protective for IPV exposure among young women in South Africa. These effects, while modest in magnitude, could represent a meaningful impact were the gamified chatbot to be scaled.