Participatory approaches to innovation aim to address persistent failures of technology to respond to end-user needs and context. Here, we present the results of a transdisciplinary project aimed at co-developing new technologies for water quality monitoring in remote locations in developing countries. Drawing from critical social science, we developed and implemented a suite of approaches to engage community members and other regional stakeholders in an innovation process that is simultaneously social and technical. Part of our community engagement activities involved the sampling and molecular analysis of drinking water sources from two communities on the island of Efate in Vanuatu. The results revealed evidence for temporal variations in the extent of faecal contamination from different sources. This analysis was used to help frame discussions about microbial contamination, water quality and health, which, along with other structured conversations, led to technical and institutional specifications for water quality sensing. These co-developed specifications were striking, contradicting widely assumed requirements for handheld, rapid, mobile devices. Informed by these specifications, a device for monitoring colorimetric changes in response to microbial growth was designed and built. This device was able to quantify growth of faecal coliform indicator species Escherichia coli inoculated into sterile media. Subsequently, we showed the device could detect E. coli inoculated into sterilised river water. The limit of detection was as low as a single E. coli cell in 100 mL of liquid. Detection at this low concentration was achieved in 16 hours, meeting a specification requirement established through the co-design process.