To investigate whether galectin‐9 could mediate immune tolerance, cocultures of human autologous immune cells and cancer organoids and a syngeneic mouse model of lung cancer xenografts were studied, which showed that tumor‐secreted galectin‐9 suppressed T‐cell toxicity through inducing the Treg fraction and promoting CD8+ T‐cell apoptosis. Here, CD8A is linked to lung cancer.