Algae　toxins　pose　a　severe　threat　to　human　health　all　over　the　world.　In　this　study,　magnetic　metal/nitrogen-doped　carbon　nanotubes　(M-NCNTs)　were　facilely　synthesized　based　on　one-step　carbonization　and　applied　for　magnetic　solid-phase　extraction　of　okadaic　acid　(OA)　from　seawater　followed　by　high　performance　liquid　chromatographic　tandem　mass　spectrometry　(HPLC-MS/MS)　analyses.　Differences　in　the　physicochemical　properties　of　the　three　prepared　materials　(Fe/Co/Ni-NCNTs)　were　investigated　to　confirm　the　best　extraction　material.　Among　them,　Ni-NCNTs　demonstrated　a　faster　extraction　rate　(10　min)　and　higher　adsorption　capacity　(223.5　mg　g−1),　mainly　due　to　the　higher　specific　surface　area,　suitable　pore　structure　and　more　abundant　pyridine　nitrogen　ring.　Under　the　optimal　conditions,　the　calibration　curve　was　linear　over　the　range　(1.0–800.0　pg　mL−1)　with　good　determination　coefficients　(R)　of　0.9992.　The　limit　of　detection　(LOD)　obtained　in　multiple　replicates　was　0.4　pg　mL−1.　Three　seawater　samples　were　measured　by　the　developed　method,　12.3　pg　mL−1　of　OA　was　detected　with　a　satisfying　recovery　(88.6%-106.7%)　and　acceptable　repeatability　(RSD　≤　4.8%,　n　=　6).　The　results　demonstrate　that　M-NCNTs　materials　are　a　promising　candidate　for　magnetic　solid-phase　extraction.　Benefiting　from　its　high　extraction　and　interference　resistance,　the　established　analytical　method　is　expected　to　be　extended　to　detect　other　marine　environmental　pollutions.　©　2022　Elsevier　B.V.