Immune evasion in lung cancer is closely associated with the dysregulation of molecular chaperones and immunoregulatory pathways. Heat shock protein 90 (HSP90), which is frequently overexpressed in lung cancer and correlates with poor prognosis, has emerged as a promising therapeutic target. Here, we investigated whether targeting the HSP90–aryl hydrocarbon receptor (AhR) axis with usnic acid (UA) could suppress immune evasion mechanisms in lung cancer. Through target prediction and molecular docking, UA-bead–based proteomic profiling, and in vitro assays, we identified HSP90 as a direct binding of UA. Unlike classical HSP90 inhibitors, UA downregulates HSP90 protein level and disrupts the HSP90–AhR complex, thereby promoting proteasomal degradation of AhR and reducing its half-life. This disruption suppresses AhR-associated gene expression and tryptophan metabolism-related markers under both AhR ligand-bound and ligand-free conditions. Additionally, the immune checkpoint molecules programmed death-ligand 1 (PD-L1) and inducible T-cell costimulator ligand (ICOSL) were markedly downregulated, demonstrating that UA modulates the tumor immune microenvironment via the HSP90–AhR axis. To assess its translational relevance, a water-soluble derivative of UA, potassium usnate (KU), was evaluated in a syngeneic lung cancer mouse model. KU treatment inhibited tumor growth in a dose-dependent manner, reduced tumor-associated macrophages and programmed cell death protein 1 (PD-1⁺) T cells, and increased the infiltration of cytotoxic T lymphocytes (CD8⁺) and helper T cells (CD4⁺). In addition, KU decreased the proliferation marker, antigen Kiel 67 (Ki67⁺), and HSP90⁺ cell populations within tumors. Together, these findings demonstrate that UA/KU targets the HSP90–AhR axis, suppresses immune evasion pathways, and offers a novel immunomodulatory approach for lung cancer therapy. Keywords Usnic acid, Heat shock protein 90, Aryl hydrocarbon receptor, Immune checkpoint molecules, Lung cancer.