Adaptive resistance to therapy is a hallmark of cancer progression. To date, it is not entirely clear how microenvironmental stimuli would mediate emergence of therapy-resistant cell subpopulations, although a rearrangement of the cancer cell secretome following therapy-induced stress can be pivotal for such a process. Here, by using the highly chemoresistant malignant pleural mesothelioma (MPM) as an experimental model, we unveiled a key contribution of the chaperone HSP90 at assisting a chemotherapy-instigated Senescence-Associated-Secretory-Phenotype (SASP). Thus, administration of a clinical trial grade, HSP90, inhibitor blunted the release of several cytokines by the chemotherapy-treated MPM cells, including interleukin (IL)-8. Reduction of IL-8 levels hampered the FAK-AKT signaling and inhibited 3D growth and migration. This correlated with downregulation of key EMT and chemoresistance genes and affected the survival of chemoresistant ALDHbright cell subpopulations. Altogether, inhibition of HSP90 provoked a switch from a pro-tumorigenic SASP to a pro-apoptotic senescence status, thus resulting in chemosensitizing effects. In mouse xenografts treated with first-line agents, inhibiting HSP90 blunted FAK activation and reduced the expression of ALDH1A3 and the levels of circulating human IL-8, these latter strongly correlating with the effect on tumor growth. We validated the above findings in primary mesothelioma cultures, a more clinically relevant model. We unveiled here a key contribution of the chaperone HSP90 at assisting the secretory stress in chemotherapy-treated cells, which may warrant further investigation in combinatorial therapeutic settings.