Family planning benefits maternal-child health, education, and economic wellbeing. Despite global efforts, an unsatisfied demand for family planning persists in sub-Saharan Africa. Based on previous successful partnerships, the aim of this study was to determine whether an educational intervention for religious leaders would increase community knowledge, demand for, and ultimately uptake of family planning. In this open-label, cluster randomised trial in Tanzania, 24 communities were randomised (1:1) to intervention or control arm. Communities, defined as the catchment area of a single public health facility, were eligible if they were at least 15 km from Mwanza City and had not previously participated in a health intervention for religious leaders. Random allocations were determined by coin toss and were not revealed to clinicians at health facilities in intervention and control communities, nor to the data entry team; however, due to the nature of the intervention, masking of religious leaders in the intervention communities was not possible. All Christian religious institutions were invited to send four leaders to an educational intervention that incorporated cultural, theological, and medical teaching about family planning. The primary outcome was contraceptive uptake at the community health facility during the year post intervention versus the year before the intervention. This trial was registered at clinicaltrials.gov, NCT03594305. 75 communities in three districts were assessed for eligibility. 19 communities were excluded and 56 were eligible for study inclusion and were placed in random order to be invited to participate. The first 24 communities that were invited agreed to participate and were randomly assigned to receive the educational intervention either during the trial or after trial completion. Between July 10, 2018 and Dec 11, 2021, we provided the intervention in 12 communities and compared contraceptive uptake with 12 control communities. All were followed up for 12 months. In intervention communities, contraceptive uptake increased by a factor of 1·47 (95% CI 1·41-1·53) in the post-intervention (prospective) versus pre-intervention (historical) year (geometric mean of contraceptive uptake, 466 in the prospective year vs 312 in the historical year), versus 1·24 (95% CI 1·20-1·29) in control communities (geometric mean, 521 in the prospective year vs 429 in the historical year). The rate of change in contraceptive uptake was greater in intervention communities (between-group ratio of geometric mean ratios over time, 1·19 [95% CI 1·12-1·25]; p<0·0001). The COVID-19 pandemic was associated with decreased contraceptive uptake (geometric mean, 365 during the pandemic in communities that had the majority of their prospective 12-month data collection periods occur after March 16, 2020, vs 494 before the pandemic; geometric mean ratio, 0·72 [95% CI 0·57-0·90]; p=0·0040). This intervention offers a scalable model, leveraging influence of trusted religious leaders to increase knowledge and uptake of family planning. New strategies such as this could help to overcome setbacks that occurred during the COVID-19 pandemic. John Templeton Foundation and Weill Cornell Medicine Dean's Diversity and Healthcare Disparity Award. For the Kiswahili translation of the abstract see Supplementary Materials section.