PURPOSE: Brain metastases are associated with high morbidity and often resistant to immune checkpoint inhibitors. We evaluated whether CDK4/6 inhibitor (CDKi) abemaciclib can sensitize intracranial tumors to PD-1 inhibition in mouse models of melanoma and breast cancer brain metastasis. EXPERIMENTAL DESIGN: Treatment response was evaluated in vivo using immunocompetent mouse models of brain metastasis bearing concurrent intracranial and extracranial tumors. Treatment effect on intracranial and extracranial tumor immune microenvironments was evaluated using immunofluorescence, multiplex immunoassays, high-parameter flow cytometry and T cell receptor profiling. Mice with humanized immune systems were evaluated using flow cytometry to study the effect of CDKi on human T cell development. RESULTS: We found that combining abemaciclib with PD-1 inhibition reduced tumor burden and improved overall survival in mice. The tumor immune microenvironment, which differed based on anatomical location of tumors, was altered with CDKi and PD-1 inhibition in an organ-specific manner. Combination abemaciclib and anti-PD-1 treatment increased recruitment and expansion of CD8+ effector T cell subsets, depleted CD4+ regulatory T (TREG) cells, and reduced levels of immunosuppressive cytokines in intracranial tumors. In immunodeficient mice engrafted with human immune systems, abemaciclib treatment supported development and maintenance of CD8+ T cells and depleted TREG cells. CONCLUSIONS: Our results highlight the distinct properties of intracranial and extracranial tumors and support clinical investigation of combination CDK4/6 and PD-1 inhibition in patients with brain metastases.