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.