The ethanol extract of Rhinachantus nasutus (L.) Kurz stem bark (RNSB) was prepared using ultrasonication. This extract has a high total phenolic content, strong antioxidant activity, and actively inhibits alpha-glucosidase activity. On the other hand, the possible cytotoxic, anti-inflammatory, and antibacterial effects of the ultrasonic extract have not been reported. The purpose of this study was to investigate the cytotoxicity of an ethanol extract of RNSB using the Brine Shrimp Lethality Test (BSLT) method with Artemia salina larvae as the test organism. In addition, anti-inflammatory activity was evaluated using the protein denaturation inhibition method, and antibacterial activity was evaluated using the paper disc method against Escherichia coli and Bacillus subtilis. This study showed that RNSB ethanolic extract had a strong cytotoxic effect with an LC50 of 10.93 ppm. The anti-inflammatory effect of this RNSB extract is also very strong, with an IC50 value of 3.00 ± 0.009 mg/L. In contrast, the RNSB extract was not effective against E. coli and B. subtilis bacteria. These findings suggest that R. nasutus stem bark may have anti-inflammatory and anti-cancer properties.