环氧合酶（英语：Cyclooxygenase，简称COX）是一种酶（又名酵素），负责合成重要的生物激素——前列腺素家族的导介物质。当身体组织受到某种刺激如外伤、感染等会活化环氧合酶，使花生四烯酸大量转变为PGE₂、 PGF_2α 及血栓素等前列腺素。

常在发炎部位，发现有大量COX-2存在。

在药理上，抑制COX可以减轻炎症的疼痛症状 ，这就是非类固醇消炎止痛药的药理方法。「前列腺素合成酶」，有时是用来指环氧化酶。

Cyclooxygenase (COX), officially known as prostaglandin-endoperoxide synthase (PTGS), is an enzyme (EC1.14.99.1) that is responsible for formation of prostanoids, including thromboxane and prostaglandins such as prostacyclin.

The abbreviation "COX" is more often encountered in medicine. In genetics, the "PTGS" symbol is officially used for the prostaglandin-endoperoxide synthase (cyclooxygenase) family of genes and proteins, because the stem "COX" was already used for the cytochrome c oxidase family of genes and proteins.