叶酸（英语：folic acid，又称为维生素B₉或者Bc。最早称作维生素M）是维生素B9的水溶形式。叶酸的名字来源于拉丁文folium（意思是叶子）。由米切尔（H．K．Mitchell，1941）及其同事首次从菠菜叶中提取纯化出来，命名为叶酸。叶酸作为重要的一碳载体，在核苷酸合成，同型半胱氨酸的再甲基化等诸多重要生理代谢功能方面有重要作用。因此叶酸在快速的细胞分裂和生长过程（如婴儿发育、怀孕）中有尤其重要的作用。叶酸能促进骨髓中的幼细胞发育成熟形成正常形态的红细胞，从而避免出血。
剑桥大学的埃里卡·沃森及其同事发现，缺乏叶酸不仅导致子一代患病，并能影响到隔代，造成同样的问题。

Folic acid (conjugate base folate) is a B vitamin. It is also referred to as vitamin M, vitamin B₉, vitamin B_c (or folacin), pteroyl-L-glutamic acid, and pteroyl-L-glutamate.

Food supplement manufacturers often use the term folate for something different from "pure" folic acid: in chemistry, folate refers to the deprotonated ion, and folic acid to the neutral molecule—which both coexist in water. The International Union of Pure and Applied Chemistry and the International Union of Biochemistry and Molecular Biology state that folate and folic acid are the preferred synonyms for pteroylglutamate and pteroylglutamic acid, respectively.