The crucian carp ( Carassius carassius ) is the only native representative of its genus inhabiting Croatian freshwater ecosystems. This limnophilic species has become increasingly rare and endangered across its entire range in recent decades. In response to reported declines of crucian carp populations across Europe, a comprehensive analysis of available data was carried out for both its native and introduced ranges in Croatia, aiming to assess the species’ spatial distribution and population trends. The geographic distribution of C. carassius in Croatia was investigated using data collected from multiple sources, including scientific literature, museum collections, citizen science platforms, and additional information provided by anglers. Literature data included scientific articles, books, and field survey reports conducted for monitoring purposes. Only records with precise locality information were included, and for scientific articles and field surveys, the date of collection was also required. Museum collection data were included when they provided detailed information on locality, collection date, and, when available, the name of the collector. Records from the citizen science platform iNaturalist were accepted only after verification of species identity from photographs and confirmation of locality and observation date. Additional records from anglers were considered only if accompanied by a photograph confirming species identification. For the assessment of present distribution, only confirmed records from the last five years were included, whereas past distribution was assessed using all relevant records available from literature, museum collections, and other sources. All validated occurrences were then aggregated into 10 × 10 km grid cells in QGIS to evaluate the spatial distribution of the species. A total of 182 confirmed localities were used to reconstruct the historical distribution, while 42 records represent the current distribution (Fig. 1). The historical distribution of C. carassius shows its presence in 91 10 × 10 km grid cells, of which 79 are located within the continental, 10 within the Alpine, and three within the Mediterranean biogeographical regions. Current distribution data indicate that the species has been recorded in 30 10 × 10 km grid cells, with 22 located in the continental and eight in the Alpine biogeographical regions. These results indicate a drastic decline in C. carassius populations in Croatia, with an estimated ~80% reduction across its natural distribution range. A higher number of records was observed in oxbow lakes in the Baranja region and in the Kopački Rit Nature Park. At almost all sites within the continental biogeographical region, C. carassius was found in association with C. gibelio , whereas at only a few sites in the continental and Alpine regions it occurred as the sole representative of the genus Carassius . Competition with C. gibelio , now widespread in Croatian freshwater habitats, appears to be the main factor driving C. carassius declines in the continental region (see Suppl. material 1). Relatively stable populations have been recorded in ponds and other watercourses of the Alpine biogeographical region, where the species was introduced. These sites represent important refugia for potential conservation and repopulation efforts within the species’ natural range. This study emphasizes the need to assess the genetic status of Croatian C. carassius populations to determine their true condition and to design effective long-term conservation strategies at both national and regional scales.