Vibrio parahaemolyticus is a naturally occurring bacterium common in coastal waters where it concentrates in shellfish through filter feeding. The bacterium is a human pathogen and the leading cause of seafood-borne gastroenteritis. Presently there is little information regarding mechanisms of environmental persistence of V. parahaemolyticus or an accurate early warning system for outbreak prediction. Vibrios have been shown to adhere to several substrates in the environment, including chitin, one of the most abundant polymers in the ocean. Diatoms are abundant in estuarine waters and some species produce chitin as a component of the silica cell wall or as extracellular fibrils. We examined the role of specific surface structures on the bacterium, the type IV pilins PilA and MshA, in adherence to diatom-derived chitin. Biofilm formation and adherence of V. parahaemolyticus to chitin is mediated by the ability of the bacterium to express functional type IV pili. The amount of adherence to diatom-derived chitin is controlled by increased chitin production that occurs in later stages of diatom growth. The data presented here suggest late-stage diatom blooms may harbour high concentrations of V. parahaemolyticus and could serve as the foundation for a more accurate early warning system for outbreaks of this human pathogen.