肾（Kidney）是脊椎动物的一种器官，属于泌尿系统的一部分，负责过滤血液中的杂质、维持体液和电解质的平衡，最后产生尿液经由后续管道排出体外；同时也具备内分泌的功能以调节血压。在人体中，正常成人具备两枚肾脏，位于腰部两侧后方，因此又称为腰子，状似拳头大小的扁豆子，尽管尺寸不大，通过肾脏的血流却占有总血量的四分之一。在生理上，肾脏主要可影响血流量、血液组成、血压调节、骨骼发育，并带有部分重要的代谢功能，因此若有相关病变可引起发育异常、水肿或脱水、免疫系统的破坏，甚至可导致死亡。

The kidneys are bean-shaped organs that serve several essential regulatory roles in vertebrates. Their main function is to regulate the balance of electrolytes in the blood, along with maintaining pH homeostasis. They also remove excess organic molecules from the blood, and it is by this action that their best-known function is performed: the removal of waste products of metabolism. Kidneys are essential to the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and regulation of blood pressure (via maintaining the salt and water balance). They serve the body as a natural filter of the blood, and remove water-soluble wastes which are diverted to the bladder. In producing urine, the kidneys excrete nitrogenous wastes such as urea and ammonium. They are also responsible for the reabsorption of water, glucose, and amino acids. The kidneys also produce hormones including calcitriol and erythropoietin. An important enzyme, renin, is also produced in the kidneys which acts in negative feedback.