核糖核酸（英语：ribonucleic acid，缩写：RNA）是一种重要的生物大分子，因为分子由核糖核苷酸组成而得名。

每个RNA分子都由核苷酸单元长链组成，每个核苷酸单元含有一个含氮碱基、一个核糖和一个磷酸基。RNA是具有细胞结构的生物的遗传消息中间载体，并参与蛋白质合成；还参与基因表达调控。对一部分病毒而言，RNA是其唯一的遗传物质。

RNA存在于一切细胞的细胞质和细胞核中，也存在于大多数已知的植物病毒和部分动物病毒以及一些噬菌体中。

Ribonucleic acid (RNA) is a polymeric molecule implicated in various biological roles in coding, decoding, regulation, and expression of genes. RNA and DNA are nucleic acids, and, along with proteins and carbohydrates, constitute the three major macromolecules essential for all known forms of life. Like DNA, RNA is assembled as a chain of nucleotides, but unlike DNA it is more often found in nature as a single-strand folded onto itself, rather than a paired double-strand. Cellular organisms use messenger RNA (mRNA) to convey genetic information (using the letters G, U, A, and C to denote the nitrogenous bases guanine, uracil, adenine, and cytosine) that directs synthesis of specific proteins. Many viruses encode their genetic information using an RNA genome.