Marine sponges-associated bacteria are among the wealthiest sources of pharmacologically active chemicals from marine organisms. Many species of endosymbiont bacteria isolated from marine sponges had been studied and showed significant bioactivity. Indonesia is very rich in marine sponges. However, only a limited number of studies have been conducted to reveal endosymbiont bacteria for their medicinal potential. This study aims to determine the antimicrobial potential of endosymbiont bacteria isolated from sponges collected from Kotok Kecil Island, Jakarta. The antimicrobial was carried out by qualitative test using the modified GIBEX method and the Kirby-Bauer disc diffusion method. The results showed that from 15 isolates of endosymbiont bacteria were obtained, and four isolates showed strong antimicrobial activity, each isolated from Petrosia sp., Petrosia nigricans, Dasychalina sp., and Cinachyrella australiensis. The fermented filtrate of isolates in Nutrient Broth media showed more potent antimicrobial activity than ethyl acetate extract. The largest average inhibition zone against Candida albicans, Escherichia coli, and Streptococcus mutans were 19.2 mm, 11.8 mm, and 9.1 mm, respectively. Based on the results, it can be concluded that endosymbiont bacteria isolated from marine sponges collected from Kotok Kecil Island has great potency to be further developed as a source of new antimicrobial compounds.