Visual methods of surveying snakes are subject to observer bias and their accuracy is often questioned. Data collection by non-specialists offers an alternative way to record sightings. We present citizen science as a valuable tool for surveying the viperid snake Lachesis rhombeata, yielding more data than a traditional visual census in a remnant of the Atlantic Forest in north-east Brazil. From visits to local museum/university collections and a literature search, we obtained only 10 georeferenced historical records for this species in the study area dating from 1990 to 2020. In 218 h of visual field surveys during 2022 and 2023, covering 15,000 m2, we did not locate any L. rhombeata individuals, probably because their low population density and cryptic habits made their detection difficult. However, during the same period we acquired 110 georeferenced records through citizen science. The high number of records from citizen science underscores the value of this approach when working with an elusive and threatened snake. Lachesis rhombeata is often implicated in human–wildlife conflict but engaging local people in the project led to positive behavioural changes. Through citizen science we were able to examine live individuals, map the species’ current distribution, investigate threats to its survival and contribute data for studies on taxonomy, diet, home range, diseases and ethno-herpetology.