This chapter presents evidence that some edible seaweeds contain substantial amounts of biologically available B12, which is lacking in other plant-derived foods. There is a significant accumulation of methylmalonic acid and homocysteine during B12 deficiency, which is usually considered indicative of the vitamin deficiency. The chapter summarizes the characterization and bioavailability of B12 compounds from edible seaweeds. Various types of edible seaweeds are available worldwide, and these are known to be rich in vitamins, minerals, and dietary fibers. Although only trace B12 amounts have been detected in some green and red algae, Enteromorpha prolifera and Porphyra yezoensis have been reported to contain substantial B12 amounts. Although B12 has been identified as a major corrinoid compound in Chinese dried products produced from Porphyra spp. using Escherichia coli 215 bioautography following thin-layer chromatography PseudoB12 has also been detected as a minor peak using liquid chromatography/electrospray ionization-tandem mass spectrometry.