Mycosporine-like amino acids (MAAs) have been identified as photoprotective agents for photosynthetic organisms, mainly against ultraviolet radiation. However, the role of these compounds associated with high intensity of photosynthetically active radiation (PAR) is not completely understood. In the present study, the MAA profile of laboratory-cultivated Gracilariopsis tenuifrons was investigated at three PAR levels. Ethanolic extract afforded a mixture of five components. Four of them were identified as asterina-330, palythinol, palythene, and usujirene. This is the first report of usujirene for Gracilariaceae. Despite the fact that the PAR increase did not alter the qualitative composition of MAAs, a clear correlation between palythinol amount and irradiance during the 7-day experiment was detected. This increase of palythinol correlated to high PAR supports the hypothesis of photoprotection related to these compounds.