Ciguatera fish poisoning (CFP) is the most common and one of the most relevant seafood-borne diseases worldwide. CFP is caused by the ingestion of fish contaminated with ciguatoxins (CTX), potent lipophilic marine toxins with complex chemical structures produced by microalgae of the genus Gambierdiscus and Fukuyoa, which are transferred and metabolised through the food webs. The importance of CTXs in seafood safety and their emerging occurrence in locations far away from the tropical areas were they have been historically found, highlight the need for alternative analytical methods for their rapid, simple and cost-effective detection. In this sense, a portable electrochemical biosensor for the detection of CTXs is presented. Two different capture antibodies able to recognise the left wing of CTX1B and 54-deoxyCTX1B and the left wing of CTX3C and 51-hydroxyCTX3C are immobilised on multi-walled carbon nanotube-modified electrodes. A sandwich configuration is adopted by the use of a biotinylated antibody that binds to the right wing of these four CTX congeners. PolyHRP-streptavidin is used as enzymatic label for signal amplification and detection of the biotinylated antibody. Amperometric currents are recorded with a small and ready-to-go potentiostat inserted in a smartphone, providing in situ measurements. A CTX1B calibration curve is obtained, achieving a limit of detection at the pg/mL level. After the evaluation of the matrix effects, the ability of the immunosensor to detect CTX1B contents at the 0.01 µg/kg guidance level proposed by the United States Food and Drug Administration (US FDA) is demonstrated. The biosensor is being applied to the analysis of naturally contaminated fish samples and results will be compared with those obtained by cell-based assay (CBA) and liquid chromatography coupled to mass spectrometry (LC/MS). This portable, easy-to-handle, rapid and low-cost analytical tool will facilitate the monitoring of CTX contents to guarantee seafood safety.