Environmental DNA (eDNA) methods are providing tools for detecting invasive species in aquatic environments. Targeted qPCR assays applied to eDNA samples promise to overcome limitations of traditional methods, especially for early detection. The European green crab (Carcinus maenas) is considered one of the most successful invasive species globally due to the large range it has invaded and negative impacts on native species, marine habitats, and shellfish industries. We developed, laboratory‐validated, and field‐tested a specific qPCR assay for the detection of green crab from eDNA samples. We also show that the assay can detect green crab in bulk DNA extracted from plankton samples. Assay design, optimization, sensitivity, and specificity testing generally followed the validation pathway recommended by the World Organization for Animal Health for assays used to manage global aquatic animal health and infectious disease. Assay specificity was verified in silico and in vitro by laboratory testing 26 nontarget species, none of which showed potential for amplification. Assay sensitivity was appropriately high, with the limit of detection approaching two gene copies/μl. The assay was field‐tested on eDNA samples collected from filtered seawater at five sites on the Pacific coast of Canada known to harbor green crab based on historical monitoring data; green crab DNA was amplified from all sites. We also present early pilot field testing of the assay done on bulk DNA extracted from plankton samples from four sites from Australia, two sites with and two sites without reported records of green crab presence. Green crab was detected at both sites with known green crab records. Significant inhibition was recorded for some plankton samples but not for eDNA samples. This is the first qPCR assay for detection of European green crab, providing researchers and managers with a valuable new tool to aid early detection and ongoing monitoring.