Monoamine oxidase A (MAOA) is a membrane-bound mitochondrial enzyme that exists in almost all vertebrate tissues, where it catalyzes the degradation of biogenic and dietary-derived monoamines. MAOA has the function of regulating neurotransmitter metabolism and is associated with anti-tumor immune responses. Most previous studies have focused on the role of MAOA in tumor cells, while more recent findings suggest that MAOA plays an equally significant role in tumor-associated immune cells. In this review, we summarize the regulatory effect of MAOA on the inhibitory tumor microenvironment. The suppressing function of MAOA on various types of tumor-associated immune cells (e.g., CD8⁺ T cells and tumor-associated macrophages) by its direct effect on monoamines and their metabolic characteristics are discussed. We propose that developing novel MAOA-inhibitor drugs and exploring multidrug-combination strategies may enhance the efficacy of immune therapy for tumors. In conclusion, MAOA may act as a novel target in tumor immunity and influence the effect of tumor immunotherapy.