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Research interests in studying the biochemical nature of selenium have increased and the importance of this element as an essential micronutrient in many organisms has been well recognized. Selenium occurs in proteins in the form of the 21st amino acid, selenocysteine (SeCys or Sec). In this review, we describe the speciation analysis of the fish-specific selenoproteins and non-proteinous selenium compounds, and the nutritional bioavailability of selenium from seafood materials. Selenium is essential to fish and shellfish. The selenoproteomes (sets of SeCys-containing proteins) of fish are greater in number than those of mammals (25 selenoproteins in humans); at 30-37 selenoproteins, the selenoproteomes of fish are among the largest known. The same core selenoprotein families are found in mammals and fish. In addition, fish have several species-specific selenoproteins [fish 15 kDa selenoprotein-like protein (Fep15), selenoprotein J and selenoprotein L] that are missing in mammals. Actually, not only proteinous selenium species like selenomethionine (SeMet) and SeCys derivatives, but also many non-proteinous organic ones were detected in fish and shellfish samples. Although the selenium contents in seafood are higher than in terrestrial foodstuffs, little is known about the chemical forms of organoselenium species in seafood. The nutritional bioavailability of selenium from seafood appears to be dependent on the fish and shellfish species and/or place where they are produced; some seafood gives rise to a high bioavailability of selenium, which is comparable to that of wheat and beef. Fish and shellfish materials are major dietary sources of selenium for the Japanese population (~60% of daily intake). Seafood materials appear to contain nutritionally effective organoselenium compounds that have not yet been chemically identified., Journal of Health Science, 57(3), pp.215-224; 2011