A new tribromoiododiphenyl ether (1) and eight known brominated diphenyl ethers (2-9) were isolated from the MeOH extract of the sponge Arenosclera sp. collected in Vietnam, using repeated open column chromatography and preparative thin layer chromatography. The chemical structure of the new compound was determined by analyses of spectroscopic (1D- and 2D-NMR, and MS) data and by comparison of our data with those reported in the literature. Compounds 1, 3, and 8 exhibited strong antibacterial activities against the Gram-positive bacteria Bacillus subtilis and Staphylococcus aureus and the Gram-negative bacterium Klebsiella pneumoniae, with MIC values ranging between 0.8-6.3 µM, while 5 and 7 only displayed activities against Grampositive bacteria, with MIC values from 0.5 to 3.1 µM. Compound 2 showed activities against the four tested bacteria, with MIC values ranging between 0.56.3 µM.