Ammonia, traditionally recognized as a toxic nitrogen waste product, has recently emerged as a significant player in diverse physiological processes and implicated in cancer biology. This review article provides an overview of the multifaceted impact of ammonia on cellular signaling pathways, energy metabolism, and tumor microenvironment dynamics, in particular its novel roles in neurotransmission, metabolic homeostasis, cancer cell proliferation, and immune modulation. Notably, ammonia accumulates within the tumor microenvironment, promoting nonessential amino acid synthesis, stimulating mTORC1 activation, promoting lipid synthesis, and impairing various immune cell functions, thereby promoting tumor progression. Furthermore, the potential dual role of ammonia as a tumorigenic factor and a cancer therapeutic target is discussed, shedding light on its complex regulatory mechanisms and clinical implications. This timely review aims to deepen our understanding of the emerging physiological and pathological roles of ammonia, offering valuable insights into its significance as a potential target for diagnostic and therapeutic interventions in cancer and beyond.