Citizen science is increasing and can complement the work of professional scientists, but the value of citizen data is often untested. We therefore compared the long-term changes to coral reefs that were detected by a professional and volunteer monitoring program, operated by University of Rhode Island (URI) staff and Reef Check volunteers, respectively. Both groups monitored reefs in the British Virgin Islands from 1997 to 2012 but mostly monitored different sites (URI 8 sites and Reef Check 4 sites). When URI staff visited the Reef Check sites to perform a side-by-side to comparison, Reef Check fish density estimates were consistently higher than those made by URI observers but benthic indicators showed better agreement. When long-term trends were compared, the two programs detected qualitatively similar trends in the % cover of live coral and coral rubble, but temporal changes in the cover of other benthic indicators were less consistent. The URI program detected a widespread increase in parrotfish densities and a decline in snappers, whereas the Reef Check surveys detected no consistent changes in any fish density indicators. Overall, site-specific temporal trends revealed by the URI program were more often statistically significant than those from Reef Check (twice as often for benthic taxa, and five times as often for fish taxa), which implies greater precision of the scientists’ counts. Nonetheless, volunteers were able to detect important changes in benthic communities and so have a valuable role to play in assessing change on coral reefs.