Pro-survival BCL‑2-family proteins are often over-expressed in tumours and essential for their sustained expansion. Moreover, the pro-survival BCL-2 family member BCL‑XL renders malignant tumour cells resistant to diverse anti-cancer therapeutics. Hence, enhancing apoptotic responses by inhibiting BCL‑XL is likely to have widespread utility in cancer treatment and, compared to inhibiting multiple pro-survival BCL-2 family members, a BCL‑XL-selective inhibitor would be expected to minimize the toxicity to normal tissues. Here, we describe the discovery by a high throughput screen of a novel series of small molecules targeting BCL‑XL and their structure-guided development by medicinal chemistry. The optimized compound, WEHI-539, has high affinity (sub-nM) and selectivity for BCL‑XL and potently kills cells by selectively antagonizing the pro-survival activity of BCL‑XL. WEHI-539 is an invaluable tool for distinguishing the roles of BCL-XL from those of its pro-survival relatives, both in normal cells and crucially in malignant tumor cells, many of which may prove to rely upon BCL‑XL for their sustained growth.