Based　on　the　catalysis　enhancement　strategy　of　Au@Pt　nanoparticles　(Au@Pt　NPs)　and　horseradish　peroxidase　(HRP)　related　to　the　TMB-H2O2　indicator,　a　sensitive　colorimetric　immunoassay　was　established　for　trace　okadaic　acid　(OA)　detection.　The　anti-OA　monoclonal　antibody　(McAb)　with　a　high　Kaff　constant　was　prepared　and　modified　on　Au@Pt　NPs.　Through　grafting　the　HRP　conjugated　goat　anti-mouse　IgG　antibody　(IgG)　on　Au@Pt/McAb,　bifunctional　composites　with　Au@Pt-Ab　and　HRP　were　prepared　and　adopted.　Characteristics　including　morphology,　specificity　and　catalytic　performance　were　evaluated.　Under　the　optimal　conditions,　the　sensitivity　of　the　resultant　enzyme　immunoassay　was　significantly　improved,　and　a　low　limit　of　detection　(LOD)　of　OA　was　achieved　at　0.04　ng　mL−1　(equivalent　to　0.6　μg　kg−1　in　mussel　tissue),　which　was　better　than　that　of　most　HRP　or　Au/HRP　enzyme-linked　immunosorbent　assays.　When　applied　to　fortified　shellfish　samples　(e.g.　oysters,　mussels　and　clams),　the　recoveries　ranging　from　98.3　±　2.3%　to　106.0　±　9.0%　were　acceptable　and　comparable　with　those　of　the　LC-MS　method.　Acceptable　precision　was　achieved　with　a　variation　coefficient　(CV)　of　2.3-8.4%.　The　method　provides　a　promising　alternative　for　the　highly　sensitive　detection　of　the　OA　marine　toxin　at　trace　levels.　©　2022　The　Royal　Society　of　Chemistry