乙酰基（英语：Acetyl），化学式为CH₃-C-或记为Ac-，是一个由甲基和羰基组成的酰基官能团。乙酰基存在于很多化合物之中，包括乙酰胆碱、乙酰辅酶A、对乙酰氨基酚、乙酰水杨酸（阿司匹林）、乙酸、乙酰氯、苯乙酮、乙酰胺和乙酸酐等。

Ac在化学中可以有很多含义。无机化学中，Ac可以指元素锕（Actinium），或乙酸根（CH₃COO，Acetate）；有机化学中，Ac一般指乙酰基（CH₃C(O)-）。因此比乙酸根多一个电子的乙酰氧基（Acetoxy）的缩写为AcO-。

向分子中引入乙酰基的化学反应称为乙酰化，相应的试剂称为“乙酰化试剂”和“乙酰转移酶”。在它们的作用下，乙酰化可以发生在辅酶A、组蛋白和其他蛋白质上，性质得以改变。

In organic chemistry, acetyl is a functional group, the acyl with chemical formula CH₃CO. It is sometimes represented by the symbol Ac (not to be confused with the element actinium). The acetyl group contains a methyl group single-bonded to a carbonyl. The carbonyl center of an acyl radical has one nonbonded electron with which it forms a chemical bond to the remainder R of the molecule. In IUPAC nomenclature, acetyl is called ethanoyl, although this term is rarely heard. The acetyl moiety is a component of many organic compounds, including acetic acid, the neurotransmitter acetylcholine, acetyl-CoA, acetylcysteine, and the analgesics acetaminophen (also known as paracetamol) and acetylsalicylic acid (better known as aspirin).