Microalgae are one of the main problems in the management of marine resources around the world. Harmful algal blooms (HABs) have significantly impacted on marine ecosystems worldwide, affecting open ocean, deep sea, and coastal ecosystems. Phycotoxins are toxic compounds produced in large quantities during HABs, particularly accumulating in shellfish, and thus capable of entering the food chain. Consuming a sufficient amount of contaminated seafood may therefore lead to intoxication in humans. Biosensors play a crucial role as efficient and reliable analytical tools for the screening of these toxic compounds. This mini-review aims to summarize the latest advancements in biosensors for detecting marine biotoxins in mussels.