Ichthyotoxins are poisonous substances found in fish. They cause poisoning accompanied by gastrointestinal and neurological disorders. Known ichthyoalliainotoxin, causing hallucinations, ichthyocrinotoxin, located in the mucus, ichthyochemotoxin - in the blood serum of eels, carp, tuna, tench and mature gonads of some fish.

Ichthyotoxins are compounds that are either toxic to fish or are toxins produced by fish. The former include euglenophycin and primnesins produced by algae, which can cause massive fish kills. The latter includes ostracitoxin produced by boxwood. Many species of toxin-producing algae can be found in both marine and freshwater environments when the algae are in bloom. Ichthyotoxic poisoning in humans can cause symptoms of varying degrees of severity, depending on the amount of toxin consumed. Symptoms of ichthyotoxin poisoning from fish poisons may include headache, vomiting, diarrhea, dizziness, and drop in blood pressure.

For example, the eggs of Lepisosteidae contain toxins. Toxin-producing algae include Prymnesium parvum.