LLL12 is a potent STAT3 inhibitor. LLL12 inhibits colony formation and cell migration and works synergistically with doxorubicin and gemcitabine. LLL12 demonstrated a potent inhibitory activity on breast and glioblastoma tumor growth in a mouse xenograft model. LLL12 may be a potential therapeutic agent for human cancer cells expressing constitutive STAT3 signaling.