Effective transnational policies regarding nuclear disarmament, climate change, and human rights are more often generated and adhered to by democracies rather than non-democracies, but electoral fraud in developing countries threatens their democratic foundations. Every year, the international community spends millions of dollars on election observation missions, yet little scientific evidence demonstrates any fraud reduction from these efforts. We conducted the first randomized controlled trial to detect and deter fraud on a nation-wide sample of polling stations in Uganda using scalable information and communications technology (ICT). In treatment stations, researchers delivered letters to polling center managers stating their tallies would be photographed using smartphones and compared against official results. Compared to control stations with no letters, the letters reduced electoral fraud by reducing the vote share for incumbent President Museveni, the candidate most likely to rig, reducing the number of sequential digits found on tallies – a fraud indicator – and increasing the frequency of posting tallies. Our results demonstrate that a cost-effective, scalable, citizen and ICT-based intervention can substantially reduce electoral fraud.