Signal transducer and activator of transcription 3 (STAT3) is phosphorylated in breast cancer cells, particularly triple-negative breast cancers (TNBCs). Therefore, the inhibition of constitutive phosphorylated STAT3 is a promising therapeutic for TNBC treatment. Recently, a series of novel STAT3 inhibitors based on natural (-)-galiellalactone have been identified to inhibit STAT3 phosphorylation at the Tyr705 residue. Interestingly, the truncation of the cyclohexene moiety of (-)-galiellalactone to [3.3] bicyclic lactone as a pharmacophoric core produced improved cytotoxic effects against TNBCs. The potent analogues 16 and 17, identified from a STAT3-mediated luciferase reporter assay, selectively inhibited the STAT3 signaling pathway without affecting STAT1 or STAT5.