Cancer has become a leading cause of death worldwide, which is responsible for 7.6 million cancer deaths according to GLOBOCAN survey conducted in 2008. The exploration of cis-platin analogues (carboplatin, lobaplatin, nedaplatin, oxaliplatin) and their incorporation to the treatment of cancer patients has further led interest in exploring metal-based anticancer drugs. The current study describes the synthesis of two new tetra-coordinated mono- and tetranuclear organotin(IV) carboxylate complexes and their in vitro anticancer studies. Each one of the complexes (1–2) has been characterized by analytical (micro- and gravimetric analysis) and spectroscopic (FTIR, ¹H, ¹³C, ¹¹⁹Sn-NMR) techniques. Furthermore, molecular structures of 1 and 2 were elucidated using X-ray crystallography. The characterization data showed that the coordination took place via oxygen atoms from the carboxylate anions to generate 1 as an organodistannoxane dimer and 2 as a mononuclear complex. Exceptionally, the NMR spectroscopic and X-ray crystallographic study showed that acetone molecules also took part in crystallizing 2. Both complexes were tested against three cancerous (colon cancer HCT 116, breast cancer MCF 7, leukemia K562) and one non-cancerous (3T3-L1) cell lines. Both complexes showed same IC₅₀ value (0.2μM) against HCT 116, whereas for the other two cancer cell lines (MCF 7 and K562) and a normal cell line (3T3-L₁), 2 showed results better than 1. Importantly, the complexes showed exceptional activity against MCF 7 and K562 cell lines and the IC₅₀ values were calculated in nanomoles (MCF 7, IC_50s=86.5 and 53.4nM;K 562, IC_50s=22.9 and 49.6nM for 1 and 2, respectively). Both, 1 and 2, showed IC₅₀ values many times better than the standard drugs (5-FU, Tamoxifen, betulinic acid and cis-platin) used. Compared to cancerous cell lines, the complexes showed mild toxicity against normal cells (3T3-L1). Overall, two remained relatively effective.