Cancer-associated inflammation develops resistance to the epidermal growth-factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) in non-small cell lung cancers (NSCLCs) harboring oncogenic EGFR mutations. Stat3-mediated interleukin (IL)-6 signaling and Smad-mediated transforming growth factor-β (TGF-β) signaling pathways play crucial regulatory roles in cancer-associated inflammation. However, mechanisms how these pathways regulate sensitivity and resistance to EGFR-TKI in NSCLCs remain largely undetermined. Here we show that signal transducer and activator of transcription (Stat)3 represses Smad3 in synergy with the potent negative regulators of TGF-β signaling, c-Ski and SnoN, whereby renders gefitinib-sensitive HCC827 cells resistant. We found that IL-6 signaling via phosphorylated Stat3 induced gefitinib resistance as repressing transcription of Smad3, whereas TGF-β enhanced gefitinib sensitivity as activating transcription of Smad3 in HCC827 cells with gefitinib-sensitizing EGFR mutation. Promoter analyses showed that Stat3 synergized with c-Ski/SnoN to repress Smad2/3/4-induced transcription of the Smad3 gene. Smad3 was found to be an apoptosis inducer, which upregulated pro-apoptotic genes such as caspase-3 and downregulated anti-apoptotic genes such as Bcl-2. Our results suggest that derepression of Smad3 can be a therapeutic strategy to prevent gefitinib-resistance in NSCLCs with gefitinib-sensitizing EGFR mutation.