Tetrodotoxin (TTX) is a potent neurotoxin responsible for many food poisoning incidents and some fatalities. Although mainly associated with the consumption of pufferfish, in recent years, TTX has been found in shellfish, particularly in Europe. In this work, a magnetic bead (MB)-based colorimetric immunoassay was applied to the detection of TTX in Pacific oysters (Crassostrea gigas), razor clams (Solen marginatus) and mussels (Mytilus galloprovincialis). Effective LODs (eLODs) for TTX of 1 μg/kg in oysters and razor clams and 3.3 μg/kg in mussels, significantly below the EFSA guidance threshold (44 μg/kg), were obtained. The strategy was applied to the analysis of naturally-contaminated Pacific oysters (Crassostrea gigas) and mussels (Mytilus edulis) from the Netherlands, and TTX was detected in all samples. The approach, which takes less than 1.5 h, proved to be useful as a rapid and simple method to detect TTX, support shellfish safety and protect consumers.