Malyngamide 3 (1) and cocosamides A (2) and B (3) were isolated from the lipophilic extract of a collection of Lyngbya majuscula from Cocos Lagoon, Guam. The planar structures of compounds 1-3 were determined by spectroscopic methods. The absolute configuration of 1 was determined by modified Mosher's method, NOESY data, and comparison with lyngbic acid (4). The absolute configurations of 2 and 3 were assigned by enantioselective HPLC analysis and comparison with the closely related compound pitipeptolide A (5). Compounds 1-3 showed weak cytotoxicity against MCF7 breast cancer and HT-29 colon cancer cells.