Developing potent molecules that inhibit Bcl-2 family mediated apoptosis affords opportunities to treat cancers via reactivation of the cell death machinery. We describe the hit-to-lead development of selective Bcl-X-L inhibitors originating from a high-throughput screening campaign. Small structural changes to the hit compound increased binding affinity more than 300-fold (to IC50 <20 nM). This molecular series exhibits drug-like characteristics, low molecular weights (M-w <450) and unprecedented selectivity for Bcl-X-L. Surface plasmon resonance experiments afford strong evidence of binding affinity within the hydrophobic groove of Bcl-X-L. Biological experiments using engineered Mcl-1 deficient mouse embryonic fibroblasts (MEFs, reliant only on Bcl-X-L, for survival) and Bax/Bak deficient MEFs (insensitive to selective activation of Bcl-2-driven apoptosis) support a mechanism-based induction of apoptosis. This manuscript describes the first series of selective small-molecule inhibitors of Bcl-X-L and provides promising leads for the development of efficacious therapeutics against solid tumors and chemoresistant cancer cell lines.