Breast cancer is one of the most prevalent malignant tumors in women, but the side effects and drug resistance limit the long-term effectiveness of existing drugs. To address these issues, we designed and synthesized a series of novel mono- and bis-indole-substituted 3-indolylbenzoquinone derivatives and evaluated their inhibitory activity against breast cancer. Among them, compound 1b demonstrated the most potent inhibitory activity against the MDA-MB-231 breast cancer cell line (IC