Radio-frequency identification (RFID) tags represent some of the smallest animal-borne technologies available. They are frequently used for understanding fine-scale associations between animals and their environments. However, currently available devices are often prohibitively expensive or difficult to customise. We present ‘WildWID’, an open-source radio-frequency identification system that can be used for detecting encounters between tags and loggers and/or integrated into more advanced experimental set-ups for triggering audio or visual playbacks following the detection of RFID tag codes. WildWID systems can be made using commercially available components (costing ~$20 USD for tags and ~$60 for loggers) and can be customised to fit project and species-specific needs. WildWID tags have been deployed on a range of species including koalas Phascolarctos cinereus, lions Panthera leo and shy albatross Thalassarche cauta for a range of research topics, including the assessment of road-crossing structures and species' responses to intraspecific competitors. WildWID is a cost-effective RFID system that can increase the accessibility of this technology for a wider range of researchers and study questions. Its use provides researchers with opportunities to mitigate ethical concerns, for example, by customising systems for species-specific conditions, and its customisability extends the range of applications where RFID systems can be used by field biologists.