The introduction of non‐native species is a major threat to biodiversity. While eradication programs of well‐established invaders are costly and hazardous for non‐target species, the early detection of a non‐native species at low density is critical for preventing biological invasions in recipient ecosystems. Recent studies reveal that environmental DNA (eDNA) is a powerful tool for detecting target species in aquatic ecosystems, but these studies focus mostly on fish and amphibians. We examine the reliability of using eDNA to detect the presence of an invasive freshwater crustacean species, the red swamp crayfish Procambarus clarkii. Species‐specific primers and probes were designed; their specificity was tested using in silico PCR simulations and against tissues of other crayfish species. Limits of detection and quantification were specified for the target DNA sequence by means of quantitative PCR amplifications on dilution series of known amount of P. clarkii DNA. The method was applied to water samples collected in 158 ponds in a French Nature Park, and results were compared to a traditional method using food‐baited funnel traps. Environmental DNA had a better detection efficiency but predominantly led to divergent results compared with the trapping method. While habitat features partly explained the failure of crayfish detection by trapping, detection by eDNA was problematic at low crayfish abundances. When P. clarkii was detected, the estimated concentrations of crayfish DNA in water samples were always below the limit of quantification for the target DNA sequence. Synthesis and applications. The combination of environmental DNA (eDNA) and conventional trapping methods is recommended to monitor the invasion by P. clarkii in small waterbodies such as ponds. However, the risk of mortality for non‐target species, notably amphibians, has to be carefully evaluated before large‐scale deployment of traps. Contrary to fish and amphibians, a low amount of extracellular DNA in water is suspected to be the major limitation for crayfish detection by molecular approaches. Current advancements in PCR technology, together with optimization of the water sampling method, promise upcoming developments of eDNA detection for aquatic invertebrate species.