A marine enzyme may be a unique protein molecule not found in any terrestrial organism, or it may be a known enzyme from a terrestrial source but with novel properties. Major sources of marine enzymes are byproducts produced as a result of fish and shellfish processing, such as the viscera, heads, skin, bones, exoskeletons and shells. This chapter first talks about biodiversity and availability of the marine sources as well as properties of marine catalysts such as salt and pH tolerance, barophilicity, and cold adaptivity. Next, it presents some useful enzymes have already been isolated and applied in the food industry, including digestive proteolytic enzymes such as gastric, serine and cysteine and extremophilic enzymes such as thiol proteases, lipases, polyphenol oxidases (PPOs), chitinolytic enzymes, muscle proteases, transglutaminase. Finally, the chapter lists some food-industry applications of marine enzymes.