The organic compound citrulline is an α-amino acid. Its name is derived from citrullus, the Latin word for watermelon, from which it was first isolated in 1930. It has the idealized formula H₂NC(O)NH(CH₂)₃CH(NH₂)CO₂H. It is a key intermediate in the urea cycle, the pathway by which mammals excrete ammonia.Citrulline is made from ornithine and carbamoyl phosphate in one of the central reactions in the urea cycle. It is also produced from arginine as a by-product of the reaction catalyzed by NOS family (NOS; EC 1.14.13.39). Arginine is first oxidized into N-hydroxyl-arginine, which is then further oxidized to citrulline concomitant with release of nitric oxide.

Citrulline in the form of citrulline malate is sold as a performance-enhancing athletic dietary supplement which was shown to reduce muscle fatigue in a preliminary clinical trial.The rind of watermelon (Citrullus lanatus) is a good natural source of citrulline.