血清素（血清张力素，英语：Serotonin，又称5-羟色胺和血清胺，简称为5-HT）为单胺型神经递质，由色氨酸经色氨酸羟化酶转化为5-羟色氨酸，再经5-羟色氨酸脱羧酶合成于中枢神经元及动物（包含人类）消化道之肠嗜铬细胞。血清素在大脑中的含量为总量的2%，有九成位于粘膜肠嗜珞细胞和肌间神经丛，参与肠蠕动的调节。与肠粘膜进入血液的5-HT主要被血小板摄取。8%-9%的位于血小板中。因为5-HT不能透过血脑屏障，故中枢和外周可视为两个独立的系统。

许多真菌与植物中皆含有血清素，而人类必须通过食物获取色氨酸。

Serotonin (/ˌsɛrəˈtoʊnᵻn, ˌsɪərə-/) or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract (GI tract), blood platelets, and the central nervous system (CNS) of animals, including humans. It is popularly thought to be a contributor to feelings of well-being and happiness.