5’-[(6-Bromo-1H-indol-3-yl)methyl]-3’-methylimidazolidine-2’,4’-dione (1), a new bromoindole alkaloid was derived from the marine sponge Hyrtios erectus (order Dictyoceratida) collected in the Red Sea. Additionally, two known indole alkaloids (2–3) were also isolated. The chemical structures of the isolated compounds were elucidated on the basis of detailed spectroscopic analysis as well as comparison with published data. Compound 1 showed weak antiproliferative activity against colorectal carcinoma (HCT-116), breast adenocarcinoma cells (MCF-7) and hepatocellular carcinoma (HepG2). Additionally, at concentration of 3 mg/mL, compound 1 displayed moderate antibacterial activity against S. aureus and E. coli. © 2018 The Japan Institute of Heterocyclic Chemistry