FABP9 (Fatty Acid-Binding Protein 9) is a cytoplasmic protein and a member of the fatty acid-binding protein superfamily. It is specifically and highly expressed in testicular tissue and plays a critical role in the differentiation of male germ cells. This protein reversibly binds hydrophobic ligands such as fatty acids and retinol, mediating intracellular lipid transport and metabolic regulation. During spermatogenesis, FABP9 maintains the attachment of the acrosome to the sperm nucleus, which is essential for fertilization. Recent studies have revealed that FABP9 is significantly overexpressed in various tumors, including prostate cancer and breast cancer, where it promotes tumor progression through lipid metabolism reprogramming and regulation of the G2/M checkpoint. Its expression is associated with poor patient prognosis, making it a potential diagnostic biomarker and therapeutic target.