Sinomenine (SIN) is a bioactive alkaloid compound extracted from a Chinese medicinal plant Sinomenium acutum. It is a multitarget antitumor natural substance. Various mechanisms have been proposed for the antitumor effects of SIN, such as direct cytotoxicity, induction of apoptosis, sensitization attenuating radiotherapy and chemotherapy, reversal of drug resistance, resistance to distant metastasis, and antiangiogenesis. SIN can be used as a tumor cell killer and an adjuvant to radiotherapy and chemotherapy. However, recent studies are mostly limited to the basic experimental stage; no systematic clinical studies have yet been reported. Therefore, this paper aimed to review the mechanism underlying the antitumor effects of SIN by consulting relevant domestic and foreign studies and to provide a relevant reference for further development, use, and exploration of SIN.