Abstract: A composition for preventing, ameliorating or treating breast cancer, the composition containing a 2,3,5-substituted thiophene compound according to one embodiment of the present disclosure has an excellent activity of inhibiting the proliferation of breast cancers, particularly triple-negative breast cancer and Herceptin-resistant breast cancer, which have high mortality because there has yet been no effective therapeutic method therefor. Thus, the composition may be advantageously used for the prevention, amelioration or treatment of breast cancer.