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In Small Island Developing States (SIDS), coastal marine resources are particularly vulnerable to overexploitation, with small-scale fisheries (SSF) providing an irreplaceable source of income and food. However, the decentralized and complex nature of SSF coupled with limited monitoring capacity hinders the design of effective measures to protect marine resources and local livelihoods. To address this knowledge gap, we implemented a participatory mobile device app-based fisheries survey in 23 coastal communities of São Tomé and Príncipe (STP). Trained fishers and fish traders collected landing data from 12,403 unique fishing trips between 2019 and 2023 and recorded vessel activity on 620 sampling days. Catch and effort varied seasonally, with species composition varying across métiers (fishing types). While fishers demonstrated the ability to switch across métiers to target different species, catches were dominated by a few abundant species, notably blue runner and flying fish. Catch biomass and species richness, as well as biomass of demersal large-bodied species, were lower on São Tomé Island compared to Príncipe Island, likely reflecting historical impacts of higher fishing effort. By combining community participation with digital tools, this approach generated high-resolution data previously inaccessible to national authorities and provided actionable insights for fisheries governance in STP. Our findings highlight the value of participatory, mobile device-based monitoring for filling knowledge gaps in SIDS, demonstrating how such approaches can guide conservation priorities while integrating resource users in fisheries management.