吡咯赖胺酸是一种自然存在而少见的编码胺基酸，其编码为UAG（琥珀），该密码子通常为终止密码子。这是人们到目前为止发现的第22种，也是最后一种编码胺基酸（第21种为硒半胱胺酸）。

Pyrrolysine (abbreviated as Pyl or O; encoded by the 'amber' stop codon UAG) is an ɑ-amino acid that is used in the biosynthesis of proteins in some methanogenic archaea and bacterium; it is not present in humans. It contains an α-amino group (which is in the protonated –NH₃ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO form under biological conditions). Its pyrroline side-chain is similar to that of lysine in being basic and positively charged at at neutral pH.