Historical species records offer an excellent opportunity to test the predictive ability of range forecasts under climate change, but researchers often consider that historical records are scarce and unreliable, besides the datasets collected by renowned naturalists. This study demonstrates the relevance of biodiversity records generated through citizen science initiatives generated outside the natural sciences academia. A Spanish geographic dictionary from the mid-19th century was used to compile over 10,000 freshwater fish records, including almost 4,000 brown trout (Salmo trutta) citations, and a historical presence-absence dataset covering over 2,000 10 × 10 km cells was constructed, which is comparable with present-day data. There has been a clear reduction in trout range in the last 150 years, coinciding with a generalized warming. The current trout distribution can be accurately predicted based on historical records and past and present values of three air temperature variables. The models indicate a consistent decline of average suitability of around 25% between 1850s and 2000s, which is expected to surpass 40% by the 2050s. The largely unexplored potential of historical species records from non-academic sources should open new pathways for long-term global change science. informacion[at]ebd.csic.es: Clavero et al(2017) Historical citizen science to understand and predict climate-driven trout decline. Proc R Soc Lond DOI: 10.1098/rspb.2016.1979