AbstractNatural killer/T‐cell lymphoma (NKTL) is an uncommon malignancy with poor prognosis and limited therapeutic options. Activating mutations of signal transducer and activator of transcription 3 (STAT3) are frequently found in patients with NKTL, suggesting that targeted inhibition of STAT3 is a potential therapeutic option for this disease. Here, we have developed a small molecule drug WB737 as a novel and potent STAT3 inhibitor that directly binds to the STAT3‐Src homology 2 domain with high affinity. In addition, the binding affinity of WB737 to STAT3 is 250‐fold higher than STAT1 and STAT2. Interestingly, WB737 is more selective for NKTL withSTAT3‐activating mutations in terms of growth inhibition and apoptotic induction when compared with Stattic. Mechanistically, WB737 inhibits both canonical and noncanonical STAT3 signaling via suppression of STAT3 phosphorylation at Tyr705 and Ser727, respectively, thereby inhibiting the expression of c‐Myc and mitochondria‐related genes. Moreover, WB737 inhibited STAT3 more potently than Stattic, resulting in a significant antitumor effect with undetectable toxicity, followed by almost complete tumor regression in an NKTL xenograft model harboring aSTAT3‐activating mutation. Taken together, these findings provide preclinical proof‐of‐concept for WB737 as a novel therapeutic strategy for the treatment of NKTL patients withSTAT3‐activating mutations.