Heat shock protein 90 (Hsp90) is a molecular chaperone for numerous client proteins, many of which are crucial for the pathogenesis of non-small cell lung cancers (NSCLCs). To date, therapeutic approaches using epidermal growth factor receptor-tyrosine kinase inhibitors (EGFR-TKIs) such as gefitinib or erlotinib for the treatment of NSCLCs have been limited due to the emergence of acquired drug resistance mainly mediated by a secondary T790M mutation in EGFR. Considering this, Hsp90 inhibition seems promising as it leads to overall degradation of the oncogenic EGFR family proteins. In this regard, the present study provides the preclinical basis for a new Hsp90 inhibitor, WK88-1, for the treatment of NSCLCs harboring the T790M mutation in EGFR. Our data revealed that inhibition of Hsp90 by WK88-1 induced overall degradation of multiple oncogenic signaling molecules including EGFR, ErbB2 and ErbB3, leading to subsequent growth arrest and apoptosis in the gefitinib-resistant H1975 cell line. In addition, treatment with WK88-1 markedly inhibited proliferation, migration and invasion in H1975 cells. Moreover, an in vivo xenograft assay indicated that WK88-1 markedly suppressed tumor growth in the H1975 xenografts, highlighting the potential efficacy of WK88-1 for overcoming gefitinib resistance in NSCLCs harboring the T790M mutation in EGFR.