The NAD-dependent protein deacetylase sirtuin 3 (SIRT3) is an enzyme localized primarily in the mitochondrion, where it modulates cellular functions such as nutrient metabolism, ATP balance, antioxidant machinery, and other mechanisms fundamental to mitochondria. SIRT3 is closely associated with the pathogenesis of diverse disorders. In particular, it plays a dual role in the development and progression of cancer. In many cancers, SIRT3 acts as an oncogene by promoting or maintaining the malignant phenotypes of neoplastic cells, including uncontrolled proliferation, resistance to apoptosis, and increased motility or invasiveness; however, SIRT3 suppresses these phenotypes in certain types of malignancy. The underlying mechanisms involve depletion of intracellular reactive oxygen species, modulation of metabolic reprogramming, and regulation of intracellular signaling responsible for cell growth and death. This review summarizes recent findings concerning the characteristics and substrates/interacting partners of SIRT3, with particular emphasis on emerging mechanisms responsible for fine-tuning cellular behaviors that potentially underlie its conflicting roles in carcinogenesis.