Five new sulfur-containing polybromoindoles, 2-methylsulfinyl-3-methylthio-4,5,6-tribromoindole (1), 3-methylsulfinyl-2,4,6-tribromoindole (2), 4,6-dibromo-2,3-di(methylsulfinyl)indole (3), 3,3'-bis(2'-methylsulfinyl-2-methylthio-4,6,4',6'-tetrabromo)indole (4), and 3,3-bis(4,6-dibromo-2-methylsulfinyl)indole (5), as well as seven known sulfur-containing polybromoindoles, 3-methylthio-2,4,6-tribromoindole (6), 3-methylthio-2,4,5,6-tetrabromoindole (7), 4,6-dibromo-2,3-di(methylthio)indole (8), 2,3-di(methylthio)-4,5,6-tribromoindole (9), 4,6-dibromo-2-methylsulfinyl-3-(methylthio)indole (10), 4,6-dibromo-2-(methylthio)indole (11), and 3,3-bis(4,6-dibromo-2-methylthio)indole (12), have been isolated from the Formosan red alga Laurencia brongniartii. The structures were elucidated by extensive spectral analysis, and their cytotoxicity against selected cancer cells was measured in vitro.