Background: Low-molecular-weight fucoidan (LMF) is widely used as a food supplement for cancer patients. However, all of the studies are in vitro or were conducted using mice. Therefore, powerful clinical evidence for LMF use is relatively weak. This study aimed to evaluate the efficacy of LMF as a supplemental therapy to chemo-target agents in metastatic colorectal cancer (mCRC) patients. Methods: We conducted a prospective, randomized, double-blind, controlled trial to evaluate the efficacy of LMF as a supplemental therapy to chemotarget agents in patients with metastatic colorectal cancer (mCRC). Sixty eligible patients with mCRC were included. Finally, 54 patients were enrolled, of whom 28 were included in the study group and 26 in the control group. The primary endpoint was the disease control rate (DCR), and secondary endpoints included the overall response rate (ORR), progression-free survival (PFS), overall survival (OS), adverse effects (AEs), and quality of life (QOL). Results: The DCRs were 92.8% and 69.2% in the study and control groups, respectively (p = 0.026), in a median follow-up period of 11.5 months. The OS, PFS, ORR, AEs, and QOL did not significantly differ between the two groups. Conclusion: This is the first clinical trial evaluating the efficacy of LMF as a supplemental therapy in the management of patients with mCRC. The results indicate that LMF combined with chemotarget agents significantly improved the DCR.