大肠杆菌（学名：Escherichia coli，通常简写：E. coli）是人和动物肠道中最著名的一种细菌，主要寄生于大肠内，约占肠道菌中的1%。是一种两端钝圆、能运动、无芽孢的革兰氏阴性短杆菌。除某些菌型能引起腹泻外，一般不致病，能合成维生素B和K，对人体有益。

其属名埃希氏菌（Escherichia）来源于其发现者Theodor Escherich。1885年当Escherish尝试找出霍乱之病原时，该学者分离出大肠杆菌，最初命名为Bacterium coli commune。大肠杆菌是肠杆菌科的一员，经常作为细菌的模式生物广泛用于科学研究。

Escherichia coli (/ˌɛʃᵻˈrɪkiə ˈkoʊlaɪ/; also known as E. coli) is a Gram-negative, facultatively anaerobic, rod-shaped bacterium of the genus Escherichia that is commonly found in the lower intestine of warm-blooded organisms (endotherms). Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in their hosts, and are occasionally responsible for product recalls due to food contamination. The harmless strains are part of the normal flora of the gut, and can benefit their hosts by producing vitamin K₂, and preventing colonization of the intestine with pathogenic bacteria. E. coli is expelled into the environment within fecal matter. The bacterium grows massively in fresh fecal matter under aerobic conditions for 3 days, but its numbers decline slowly afterwards.