Amanitin (AMA) is responsible for human fatalities after ingestion of poisonous mushrooms, thus, a rapid and accurate detection method is urgently needed. Here, gold nanoparticle-based immunosensor with monoclonal antibody against AMA was constructed for rapid detection of alpha- and beta-amanitin (α- and β-AMA) in mushroom, serum and urine samples. Under optimized conditions, the visual limits of detection (vLOD) and calculated LOD for α-AMA and β-AMA in mushroom were 10 ng/g, 20 ng/g, and 0.1 ng/g, 0.2 ng/g, respectively. Analysis of wild mushroom samples was also performed using a strip scan reader in the 10 min range. Furthermore, in mushrooms containing amatoxins results were confirmed and compared with those determined by liquid chromatography tandem mass spectrometry. Thus, this immunosensor provided a useful monitoring tool for rapid detection and screening of mushroom samples and in serum and urine from subjects who accidentally consumed AMA-containing mushrooms.