$eta$-Glucuronidases of intestinal bacteria are capable of retoxifying compounds that have been detoxified by liver glucuronidation, which is one of the most important detoxication processes in the liver. Therefore, this enzyme is known to accelerate colon cancer invasion and metastasis. Two bromophenols, 2,4,6-tribromophenol (I) and 2,4-dibromophenol (II), were purified from the red alga Grateloupia elliptica. $IC_{50}$ values of bromophenol I and II against Escherichia coli $eta$-glucuronidase were 5.4 and 8.5 mg/mL, respectively. Hence, bromophenols of G. elliptica, a potent $eta$-glucuronidase inhibitor, can be used as a novel pharmaceutical agent for the prevention and treatment of colon cancer.