<p>As a marine bioresource, seaweed is a promising substance capable of delivering bioactive components. We have previously known that fermentation can change the content of bioactive compounds in seaweed and their corresponding structures, thereby enhancing their bioactivities. This paper reviews recent research on the in vitro fermentation of edible seaweeds, describes the strains mainly used for seaweed fermentation, and discusses the interrelationship between the effects of fermentation on the content and structure of functional and harmful factors as well as the mechanism of changes in bioactivities in seaweeds. Fermentation of abundant terrestrial resources has become an important means of obtaining new value-added compounds, with significant impacts on the food industry, the medical industry, and agriculture, and the exploration of seaweed fermentation technologies contributes to the sustainable development of marine resources.</p> Graphical Abstract <p></p>