Effective cancer immunotherapy is hindered by immunosuppressive crosstalk within the tumor microenvironment. We engineered a tumor immune microenvironment-on-a-chip (TIMoC) that recapitulates the vascularized, hypoxic, and spatially organized niche of human solid liver tumors. We employed TIMoC to dissect the reciprocal interaction between macrophages and natural killer (NK) cells. Macrophages induced NK cell dysfunction, while dysfunctional NK cells promoted M2 macrophage polarization. This bidirectional impairment created a self-perpetuating immunosuppressive loop. TIMoC served as an in vitro screening tool, confirming the limited efficacy of TIGIT blockade in a multicellular context and revealing synergistic anti-tumor activity for combinations of a macrophage-reprogramming agent (resiquimod) with NK cell-targeting antibodies. By incorporating patient-derived organotypic tumor spheroids and autologous immune cells, the personalized TIMoC platform modeled patient-specific responses and evaluated effective drug combinations, demonstrating its potential to guide precision immunotherapy. This work elucidates a key immunosuppressive axis and introduces a versatile platform for rationally designing combination immunotherapies.