This article reports on the participatory design of a smart forest project in a state forest reserve in the Brazilian Amazon in collaboration primarily with local community residents and secondarily with forest reserve managers and environmental scientists. The Smart Forest project was collectively proposed to use low‐cost digital technologies for forest‐carbon monitoring and sustainable development. Our research includes a feasibility test and impact assessment of the proposed components of the Smart Forest. It combines ethnographic fieldwork methods, such as interviews and focus groups on using recycled smartphones and sound‐recognition algorithms to remotely detect illegal logging, with innovative methods, such as field experiments using an open‐source smartphone app for participatory mapping of local traditional ecological knowledge (TEK). Our research findings, including unexpected ones, are helpful for community development, continuous participatory design, and academic research. This research was developed based on the first author Zhang's multiyear ethnographic field research in Amazonia and on the second author de Silva's local life experiences and action‐research interests.