Water scarcity is a global problem that can be compounded by inefficient water management, including underinvestment in infrastructure, underpricing of water use, and underenforcement of user rules. Here, we explore whether these inefficiencies can be reduced in rural Costa Rica via an externally driven community monitoring program (i.e., a program initiated by an outside organization and run by citizens). The monitoring program aimed to reduce groundwater extraction from aquifers, as well as to improve water quality and user satisfaction, by supplying additional information about field conditions and additional scrutiny of user and management authority activities and by fostering citizen engagement in water management. Using a specially designed smartphone application (app) and WhatsApp, monitors could report weekly on the conditions of the water system, including service disruptions, water quality, leaks, and source contamination. The app automatically compiled the individual reports into a summary report, which was then made available to the community water management committees and water users. The program was randomly implemented in 80 of 161 communities that expressed an interest in participating. One year after the program started, we detect modest, albeit imprecisely estimated, effects of the program in the predicted directions: less groundwater extracted, better water quality, and more satisfied users. Although the estimated effects are imprecise, the monitoring program appears to be equally or more cost effective for reducing groundwater extraction than another program in the same region that encouraged households to adopt water-efficient technologies.