<p>Bladder cancer, particularly muscle-invasive disease, has a high metastatic potential and limited treatment options, highlighting the need for new therapeutic targets. CD276 (B7-H3), a type I transmembrane protein of the B7 family, is traditionally considered an immunomodulatory ligand with an unidentified receptor. Here, we investigated the role of CD276 in bladder cancer and sought to identify its binding partner. Using a high-throughput human proteome microarray, we identified the secreted lectin galectin-1 (LGALS1) as a high-affinity binding partner of CD276, suggesting that CD276 may function as a receptor. Mechanistically, we uncovered a novel LGALS1-CD276 axis in endothelial cells, where CD276 serves as a functional receptor for tumor cell-derived LGALS1, with their interaction mediated by N-linked glycosylation at the N433 site within the D4 domain of CD276. This interaction activates the MAP4-dependent PI3K/AKT signaling pathway, thereby promoting angiogenesis and bladder cancer progression. Disruption of the LGALS1-CD276 interaction or inhibition of the downstream MAP4/PI3K/AKT pathway markedly suppressed endothelial proliferation, migration, and tube formation. In subcutaneous and orthotopic bladder cancer mouse models, anti-CD276 monoclonal antibody treatment significantly inhibited tumor angiogenesis, delayed tumor growth, and extended survival. Consistently, Cd276^-/- and/or Lgals1^-/- mouse models confirmed that the tumor-promoting effect of LGALS1 depends on CD276 expression. Clinically, elevated CD276 and LGALS1 expression was associated with poor prognosis and positively correlated with angiogenesis. Together, our findings identify CD276 as a functional cell-surface receptor and demonstrate that LGALS1 binding to CD276 promotes tumor angiogenesis through MAP4-mediated activation of the PI3K/AKT signaling pathway. These findings establish a new mechanistic foundation for bladder cancer therapy and highlight the potential of targeting the LGALS1-CD276 axis for anti-angiogenic treatment.</p>