We optimized our previously reported proline-based STAT3 inhibitors into an exciting new series of (R)-azetidine-2-carboxamide analogues that have sub-micromolar potencies. 5a, 5o, and 8i have STAT3-inhibitory potencies (IC50) of 0.55, 0.38, and 0.34 μM, respectively, compared to potencies greater than 18 μM against STAT1 or STAT5 activity. Further modifications derived analogues, including 7e, 7f, 7g, and 9k, that addressed cell membrane permeability and other physicochemical issues. Isothermal titration calorimetry analysis confirmed high-affinity binding to STAT3, with KD of 880 nM (7g) and 960 nM (9k). 7g and 9k inhibited constitutive STAT3 phosphorylation and DNA-binding activity in human breast cancer, MDA-MB-231 or MDA-MB-468 cells. Furthermore, treatment of breast cancer cells with 7e, 7f, 7g, or 9k inhibited viable cells, with an EC50 of 0.9-1.9 μM, cell growth, and colony survival, and induced apoptosis while having relatively weaker effects on normal breast epithelial, MCF-10A or breast cancer, MCF-7 cells that do not harbor constitutively active STAT3.