三羧酸循环（英语：tricarboxylic acid cycle, TCA cycle）——又称柠檬酸循环（citric acid cycle）摩天轮循环 （ferris wheel）或克雷布斯循环（Krebs cycle，简称克氏循环）——是需氧生物体内普遍存在的代谢途径，因为在这个循环中几个主要的中间代谢物是含有三个羧基的柠檬酸，因此得名；或者以发现者克雷布斯命名。三羧酸循环是三大营养素（糖类、脂类、蛋白质）的最终代谢通路，又是糖类、脂类、蛋白质代谢联系的枢纽。

在三羧酸循环中，反应物葡萄糖或者脂肪酸会变成乙酰辅酶A。这种「活化醋酸」（一分子辅酶和一个乙酰基相连），会在循环中分解生成最终产物二氧化碳并脱氢，质子将传递给辅酶烟酰胺腺嘌呤二核苷酸（NAD）和黄素腺嘌呤（FAD），使之成为NADH + H和FADH₂。NADH + H和FADH₂会继续在呼吸链中被氧化成NAD和FAD，并生成水。这种受调节的「燃烧」会生成ATP，提供能量。

The citric acid cycle – also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle – is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats and proteins into carbon dioxide and chemical energy in the form of guanosine triphosphate (GTP). In addition, the cycle provides precursors of certain amino acids as well as the reducing agent NADH that is used in numerous other biochemical reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest established components of cellular metabolism and may have originated abiogenically.