Throughout Europe, migration-related health inequalities are mirrored by large inequalities in health coverage. There is a need to develop novel strategies to secure access to health insurance for immigrants in Europe, in order to meet the shared Sustainable Development Goal of universal health coverage. We evaluated the impact of an original health-related empowerment intervention on access to health coverage among vulnerable, mostly undocumented immigrants in France. As part of the MAKASI study, we adopted an outreach approach and developed a community-based intervention with and for immigrants from sub-Saharan Africa living in precarious conditions in the Greater Paris area. This participatory intervention was grounded in the theory of individual empowerment. Using a stepped wedge randomised design, we first conducted a robust evaluation of the effect of the intervention on access to health coverage at three and six months post-intervention. We then investigated whether the intervention effect was mediated by a health empowerment process. Between 2018 and 2021, a total of 821 participants - 77% of whom were men - were recruited in public spaces and followed up for six months. Participants had been living in France for four years on average, 75% of them had no residence permit, and 44% had no health coverage at the time of inclusion. The probability of accessing health coverage increased by 29 percentage points at six months post-intervention (p < 0.01). This improvement was partially mediated by a health empowerment process, namely a reinforcement of participants' knowledge of and capacity to access available social and health resources. A health empowerment intervention largely improved access to health insurance among vulnerable immigrants in France. Our findings may be transferred to other settings where immigrants are entitled to health insurance. This study offers promising perspectives - beyond information provision and direct referral - to reduce migration-related inequalities in health coverage.