Galectin-1, a member of the galectin family of proteins, is a versatile and multifunctional molecule that plays pivotal roles in various biological processes. Galectin-1 supports tumor growth and metastasis by promoting tumor angiogenesis, inhibiting T-cell-mediated cytotoxic immune responses, and other mechanisms. The expression or overexpression of Galectin-1 in tumor and/or tumor-adjacent tissues serves as a marker of malignant progression and a potential diagnostic, prognostic, and therapeutic biomarker for cancer. Galectin-1 exhibits pleiotropic binding activities, modulating diverse immune cells primarily to promote immune tolerance and downregulate innate and adaptive immune responses. Galectin-1 is upregulated during inflammation and infection, modulating immune cells under both physiological and pathological conditions. It controls immune cell homeostasis and regulates acute and chronic inflammation by inhibiting the synthesis of proinflammatory cytokines, participating in T cell apoptosis programs, promoting the expansion of regulatory T cells (Tregs), and inactivating antigen-presenting cells. Galectin-1 is also essential for a healthy pregnancy and serves as a valuable biomarker for early diagnosis of preeclampsia (PE).