Cisplatin was the first metal-based therapeutic agent approved for the treatment of human cancers, but its clinical activity is greatly limited by tumor drug resistance. This work utilized the parent complex [Ru(phen)2(PIP)](2+) (1) to develop three Ru(II) complexes (2-4) with different positional modifications. These compounds exhibited similar or superior cytotoxicities compared to cisplatin in HeLa, A549 and multidrug-resistant (A549R) tumor cell lines. Complex 4, the most potent member of the series, was highly active against A549R cancer cells (IC50 = 0.8 μM). This complex exhibited 178-fold better activity than cisplatin (IC50 = 142.5 μM) in A549R cells. 3D multicellular A549R tumor spheroids were also used to confirm the high proliferative and cytotoxic activity of complex 4. Complex 4 had the greatest cellular uptake and had a tendency to accumulate in the mitochondria of A549R cells. Further mechanistic studies showed that complex 4 induced A549R cell apoptosis via inhibition of thioredoxin reductase (TrxR), elevated intracellular ROS levels, mitochondrial dysfunction and cell cycle arrest, making it an outstanding candidate for overcoming cisplatin resistance.