砂眼衣原体（CT, Chlamydia trachomatis OR C.trachomatis）是一种「绝对寄生病原体」（obligate intracellular pathogen），属革兰氏阴性（Gram negative）微生物（用革兰氏染色法不能将其细胞壁染成紫色），可导致砂眼，还会引致人类泌尿生殖系统的感染，女性的生殖系统比男性更容易受到感染。它的大小只有普通细菌的十分之一，特征跟病毒类似，可通过细菌过滤器，以及不能独立完成生物的代谢活动，不能用普通的琼脂培养皿培养。因此曾有一些科学家将砂眼衣原体定义为病毒。

Chlamydia trachomatis (/kləˌmɪdiə/ /trəˈkoʊmətᵻs/), also known as Chlamydia, is an organism responsible for the most prevalent STD in the United States, is one of four bacterial species in the genus Chlamydia. Chlamydia is a genus of pathogenic bacteria that are obligate intracellular parasites. C. trachomatis is a gram-negative bacterium. It is ovoid in shape and non-motile. The bacteria are non-spore-forming, but the elementary bodies act like spores when released into the host. The inclusion bodies of Chlamydia trachomatis were first described in 1942; the Chlamydia trachomatis agent was first cultured in the yolk sacs of eggs by Professor Tang Fei-fan et al in 1957.