Tumor-associated macrophages (TAMs) and other myelomonocytic cells are implicated in regulating responsiveness to immunotherapies, including immune checkpoint inhibitors (ICIs) targeting the PD-1/PD-L1 axis. We have developed an ex vivo high-throughput approach to discover modulators of macrophage-mediated T cell suppression, which can improve clinical outcomes of ICIs. We screened 1,430 Food and Drug Administration (FDA)-approved small-molecule drugs using a co-culture assay employing bone-marrow-derived macrophages (BMDMs) and splenic-derived T cells. This identified 57 compounds that disrupted macrophage-mediated T cell suppression. Seven compounds exerted prominent synergistic T cell expansion activity when combined with αPD-L1. These include four COX1/2 inhibitors and two myeloid cell signaling inhibitors. We demonstrate that the use of cyclooxygenase (COX)1/2 inhibitors in combination with αPD-L1 decreases tumor growth kinetics and enhances overall survival in triple-negative breast cancer (TNBC) tumor models in a CD8+ T cell-dependent manner. Altogether, we present a rationalized approach for identifying compounds that synergize with ICI to potentially enhance therapeutic outcomes for patients with solid tumors.