STAT3-IN-35 is a STAT3 inhibitor that binds to SH2 domain. STAT3-IN-35 inhibits the phosphorylation of STAT3 and possesses antiproliferative activities against triple-negative breast cancer (TNBC) cell lines. STAT3-IN-35 also has a toxicity and potent antitumor activity in a TNBC xenograft model.