Background: Cancer is defined as the abnormal proliferations of cells which could occur in any tissue and can cause life-threatening malignancies with high financial costs for both patients and health care system. Plant-derived secondary metabolites are shown to have positive role in various diseases and conditions. The aim of the present study is to summarize clinical evidences on the benefits of phytochemicals as adjuvant therapy along with conventional anticancer therapies. Methods: Electronic databases including Pubmed, Scopus and Cochrane library were searched with the keywords “chemotherapeutic”, “anticancer”, “antineoplastic” or “radiotherapy” with “plant”, “extract”, “herb”, or “phytochemical”, until July 2015. Only clinical studies were included in this review. Results: The findings showed that positive effects of phytochemicals are due to their direct anticarcinogenic activity, induction of relief in cancer complications, as well as their protective role against side effects of conventional chemotherapeutic agents. Results obtained from current review demonstrated that numerous phytochemical agents from different chemical categories including alkaloid, benzopyran, coumarin, carotenoid, diarylheptanoid, flavonoid, indole, polysaccharide, protein, stilbene, terpene, and xanthonoid possess therapeutic effect in patients with different types of cancer. Polyphenols are the most studied components. Curcumin, ginsenosides, lycopene, homoharringtonine, aviscumine, and resveratrol are amongst the major components with remarkable volumes of clinical evidence indicating their direct anticancer activities in different types of cancer including hepatocarcinoma, prostate cancer, leukemia and lymphoma, breast and ovarian cancer, and gastrointestinal cancers. Cannabinoids, cumarin, curcumin, ginsenosides, epigallocatechin gallate, vitexin, and salidroside are phytochemicals with significant alleviative effect on synthetic chemotherapy- induced toxicities. Conclusion: There is lack of evidence from clinical trials in case of a large number of phytochemicals and further human studies are recommended to confirm the role of plant metabolites in the management of cancer.