Objective To evaluate the efficacy and safety of melatonin as an adjuvant therapy for the tumor patients receiving chemotherapy or radiotherapy. Methods Such databases as MEDLINE (1980 to Jan. 2010), The Cochrane Library (Issue 4, 2009), WanFang Data (1980 to Jan. 2010), CBM (1980 to Jan. 2010), CNKI (1980 to Jan. 2010), ELSEVIER ScienceDirect (SDOS, 1980 to Jan. 2010), Nature (1980 to Jan. 2010) and ongoing clinical trials (www.clinicaltrials.gov and www.controlled-trials.com) were searched to collect randomized controlled trials (RCTs). The data were extracted and the quality of the included RCTs was assessed by two reviewers. Then meta-analyses were performed by using Stata 10.1 software. Results Eight RCTs were included. The results of meta-analyses showed that melatonin significantly improved the remission rate for tumor patients (RR=1.98, 95% CI 1.52 to 2.58) and the one-year survival rate (RR=1.90, 95%CI 1.28 to 2.83), and significantly reduced the toxic effects of bone marrow suppression caused by chemotherapy or radiotherapy (RR=0.12, 95%CI 0.06 to 0.27). No reports of adverse events were associated with melatonin. Conclusion The existing evidence reveals that the melatonin, as an adjuvant therapy drug for tumor, plays a certain role in improving disease remission rate, reducing the toxicity of chemotherapy and radiotherapy, and prolonging the life. It requires more high-quality RCTs for further verification because of the limitation of the included studies.