Updated project metadata. Exclusion of cancer patients with brain metastases from clinical trials is a major cause of the limited therapeutic options available for secondary brain tumors. Here, we report a novel drug-screening platform (METPlatform) based on organotypic cultures that allows identifying anti-metastatic compounds in a preparation that includes the tumor microenvironment. By applying this approach to brain metastasis, we identified HSP90 as a promising therapeutic target. A blood-brain barrier permeable HSP90 inhibitor showed high potency against mouse and human brain metastases from melanoma, lung and breast adenocarcinoma with distinct oncogenomic profiles at clinically relevant stages of the disease, including a novel model of local relapse after neurosurgery. Furthermore, in situ proteomic analysis of brain metastases treated with the chaperone inhibitor revealed non-canonical clients of HSP90 as potential novel mediators of brain metastasis and actionable mechanisms of resistance driven by autophagy. Our work validates METPlatform as a potent resource for metastasis research integrating drug-screening and unbiased omic approaches that is fully compatible with human samples. We envision that METPlatform could be established as a clinically relevant strategy to personalize the management of metastatic disease in the brain and elsewhere.