Red algae (Kappaphycus alvarezii) is a multicellular algae that has bioactive compounds that play a role in the pharmaceutical field, one of which is as an anticáncer. The purpose of this study was to determine the level of toxicity of the methanolic extract of K. alvarezii on Artemia salina Leach shrimp larvae and to determine the class of active compounds contained in the crude methanolic extract of K. alvarezii. The procedure of this research includes sample preparation, extraction, phytochemical test, and BSLT. The results of this study indicate that the methanolic extract of K. alvarezii is highly toxic to A. salina Leach with an LC50 value of 4.62 ppm, so that it has the potential as an anticancer. The active compounds contained in the extract base on phytochemical test are alkaloids, saponins, and triterpenoids. © 2023 Author(s).