The rhenium(I)-diselenoether complex (Re-diSe) is a rhenium tricarbonyl-based drug chelated by a diselenoether ligand. In this work, we compared its inhibitory effects on the hormone-independent MDA-MB231cancer line and other different cancer cell lines after an exposure time of 72 h by MTT assays. The sensitivity of MDA-MB231 was in the same range than the hormone-dependent MCF-7 breast cancer, the PC-3 prostate and HT-29 colon cancer cells, while the A549 lung and the HeLa uterine cancer cells were less sensitive. We compared the inhibitory effects of Re-diSe and of its diselenide ligand (di-Se) on MDA-MB231 and a normal HEK-293 human embryonic cell line, after 72 h and 120 h of exposure. The cytotoxicity was also studied by flow cytometry using ethidium bromide assays, as well as the effects on the ROS production by DFCA-test, while the levels of TGF-β1, VEGF-A, IGF-1 were addressed by ELISA tests. The dose required to inhibit 50% of the proliferation (IC50) of MDA-MB231 breast cancer cells decreased with the time of exposure to 120 h, while the free ligand (di-Se) was found poorly active, demonstrating the important role of Re in this Re-diSe combination. The cytotoxic effects of Re-diSe were highly selective for cancer cells, with a significant increase of the number of dead cancer cells at 5 μM for an exposure time of 120 h, while normal cells were not affected. A remarkable and significant decrease of the production of ROS together with a decrease of VEGF-A, TGF-β1, and IGF-1 by the cancer cells were also observed when cancer cells were exposed to Re-diSe.