Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The HTLV-1 genome encodes the Tax protein that plays essential regulatory roles in HTLV-1 replication and oncogenic transformation of T lymphocytes. Despite intensive study of Tax, how Tax interfaces with host signaling pathways to regulate virus replication and drive T-cell proliferation and immortalization remains poorly understood. To gain new insight into the mechanisms of Tax function and regulation, we used tandem affinity purification and mass spectrometry to identify novel cellular Tax-interacting proteins. This screen identified heat shock protein 90 (HSP90) as a new binding partner of Tax. The interaction between HSP90 and Tax was validated by coimmunoprecipitation assays, and colocalization between the two proteins was observed by confocal microscopy. Treatment of HTLV-1-transformed cells with the HSP90 inhibitor 17-DMAG elicited proteasomal degradation of Tax in the nuclear matrix with concomitant inhibition of NF-κB and HTLV-1 long terminal repeat (LTR) activation. Knockdown of HSP90 by lentiviral shRNAs similarly provoked a loss of Tax protein in HTLV-1-transformed cells. Finally, treatment of HTLV-1-transformed cell lines with 17-DMAG suppressed HTLV-1 replication and promoted apoptotic cell death. Taken together, our results reveal that Tax is a novel HSP90 client protein and HSP90 inhibitors may exert therapeutic benefits for ATL and HAM/TSP patients.