Background Knowledge of one’s HIV status is the gateway to treatment and prevention, but remains low among young people. We investigated the early impact (2016–2017) of Determined, Resilient, Empowered, AIDS-free, Mentored and Safe (DREAMS), a multisectoral HIV prevention package, on knowledge of HIV status among adolescent girls and young women (AGYW).Methods In 2017, randomly selected AGYW were enrolled into surveys, N=1081 aged 15–22 years in Nairobi slum settlements, and N=2174 aged 13–22 years in rural KwaZulu-Natal. We estimated the causal effect of being a DREAMS beneficiary on knowledge of HIV status (those who self-reported as HIV-positive or tested HIV-negative in the past year), accounting for an AGYW’s propensity to be a DREAMS beneficiary.Results In Nairobi, knowledge of HIV status was higher among DREAMS beneficiaries compared with non-beneficiaries (92% vs 69%, adjusted OR=8.7; 95% CI 5.8 to 12.9), with DREAMS predicted to increase the outcome by 28%, from 65% if none were a DREAMS beneficiary to 93% if all were beneficiaries. The increase attributable to DREAMS was larger among younger participants: 32% and 23% among those aged 15–17 and 18–22 years, respectively. In KwaZulu-Natal, knowledge of status was higher among DREAMS beneficiaries aged 13–17 years (37% vs 26% among non-beneficiaries), with a 9% difference due to DREAMS (95% CI 4.8% to 14.4%), and no evidence of effect among 18–22 years (−2.8%; 95% CI −11.1% to 5.7%).Conclusion DREAMS substantially increased knowledge of HIV status among AGYW in Nairobi, and among younger but not older AGYW in KwaZulu-Natal. Adolescent girls can be reached early (before age 18) with community-based HIV testing programmes in diverse high-prevalence settings, with a large impact on the proportion who know their HIV status.Data are available on reasonable request. Anonymised data are available upon reasonable request. Requests can be made to the LSHTM Data Compass Repository: researchdatamanagement@lshtm.ac.uk.