甲硫胺酸（英语：Methionine，又称蛋胺酸），在所有后生动物中它是一种必需氨基酸。与半胱氨酸一起，甲硫氨酸是两个含硫蛋白原氨基酸之一。对人而言是唯一的含硫必需氨基酸，有L型及D型两种，与生物体内各种含硫化合物（如：蛋白质）的代谢密切相关。是体内活性甲基和硫的主要来源。

DL-蛋氨酸可利用化学法生产。蛋氨酸是强肝解毒剂、促进发育剂，当缺乏甲硫氨酸时，会引起食欲减退。甲硫氨酸广泛应用于营养补充与畜产饲料，由于甲硫氨酸容易被鸡吸收而转变为鸡肉蛋白，在鸡饲料中添加甲硫氨酸，可少耗饲料，并使鸡肉生长健全。目前甲硫氨酸主要有四类：固体甲硫氨酸、液态羟基甲硫氨酸（MHA）、液体甲硫氨酸钠和固体羟基甲硫氨酸钙，其中固体甲硫氨酸的市场最大。但在美国甲硫氨酸市场，液态羟基甲硫氨酸（MHA）为第一大。

Methionine (abbreviated as Met or M; encoded by the codon AUG) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH₃ form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO form under biological conditions), and an S-methyl thioether side chain, classifying it as a non-polar, aliphatic amino acid. It is essential in humans, meaning the body cannot synthesize it and thus it must be obtained from the diet.