Marine algae contain various biologically active compounds. Some of them have been found to be characteristic of their chemical properties; in particular, many polysaccharides and lipids have been reported to have biological activity. However, there are few reports about low-molecular-weight compounds such as amino acids or peptides having biological activity. We have been searching for low-molecular-weight compounds having biological activity from extracts of seaweed. Three new indole derivatives having anti-photo-oxidative activity were isolated from the seaweed, Martensia denticulata. The gross structures of these new compounds were identified as [4-[[N-[2-(trimethylammonio)-3-phenylpropionyl]tryptophyl]oxymethyl]phenyl]sulfate, [4-[[N-[2-(methylammonio)-4 methylhexanoyl]tryptophyl]oxymethyl]phenyl]sulfate, and [4-[[N-[2-(dimethylammonio)-4-methylhexanoyl]tryptophyl]oxymethyl]phenyl]sulfate, which are named Denticin A, B, and C, respectively. This paper describes the isolation and identification of these three new indole derivatives having anti-photo-oxidative activity from the marine red alga, Martensia denticulata.