Abstract: Proprotein convertase subtilisin/kexin type 9 (PCSK9) is the ninth member of the proprotein convertase (PC) family. Its main function is binding to low-density lipoprotein receptors (LDLRs) on the cell membrane. The binding mediates the degradation of LDLRs and subsequent regulation of plasma LDL levels. Increasing evidence from recent studies indicates that PCSK9 plays an important role in the occurrence and development of tumors by regulating tumor cell proliferation, apoptosis, invasion, and migration. PCSK9 is also closely related to the infiltration and function of immune cells, such as CD8⁺T lymphocytes in the tumor microenvironment, and the migration and polarization of M2 macrophages. PCSK9 can be used as a target for anti-tumor therapy and combined chemotherapy or immunotherapy. Its mechanism is associated with the regulation of apoptosis factors, endoplasmic reticulum stress, mitochondrial apoptosis of tumor cells, and lipid metabolism of immune cells. This article reviews the roles and mechanisms of PCSK9 in cancer and its potential value in cancer immunotherapy.