AbstractDiketopiperazines (DKPs), which are cyclic dipeptides, have been detected in a variety of natural resources. Recently, the interest in these compounds increased significantly because of their remarkable bioactivity. This review deals with the chemical structures, biosynthetic pathways, and biological activities of DKPs from marine microorganisms, sponges, sea stars, tunicates (ascidians), and red algae. The literature has been covered up to December 2008, and a total 124 DKPs from 104 publications have been discussed and reviewed. Some of these compounds have been found to possess various bioactivities including cytotoxicity, and antibacterial, antifungal, antifouling, plant‐growth regulatory, and other activities.