Selenocysteine is an amino acid that is present in several selenoproteins (seleno-enzymes) in plants and animals (for example glutathione peroxidases, tetraiodothyronine 5' deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases and some hydrogenases).

Selenocysteine has a structure similar to cysteine, but with an atom of selenium taking the place of the usual sulfur. Selenium is incorporated into amino acid sequences of selenoproteins by the specific codon to SeCys residue.