Background: Early biomarkers of therapeutic responses could help optimize the treatment of metastatic colorectal cancer (mCRC). This prospective study was designed to explore the serial changes in plasma-circulating tumor DNA (ctDNA) as an early marker of therapeutic response to systemic treatment in mCRC. Methods: Twenty-six mCRC patients receiving standard first-line therapy once every two weeks were enrolled. Plasma ctDNA, carcinoembryonic antigen (CEA), and carbohydrate antigen 19-9 (CA19-9) were assessed serially before each of the first four cycles. Somatic mutations in plasma ctDNA were detected via next-generation sequencing using a panel of 50 cancer-related genes, and the mutation of maximal frequency in pretreatment ctDNA was selected as the candidate mutation for analysis. Radiologic responses were assessed after the fourth cycle. Results: Mutations in pretreatment ctDNA could be detected in 25 (96.2%) of the 26 initially enrolled patients. Among the 20 patients monitored serially, changes in ctDNA could differentiate patients with progressive disease two cycles (approximately four weeks) earlier than the changes in CEA and CA19-9 levels could, and changes in ctDNA levels as early as prior to cycle 2 predicted the radiologic responses after cycle 4. A log2 value of fold-change in ctDNA after cycle 1 (log2 (C1/C0)) > 0.044 predicted progressive disease, with a sensitivity and specificity of 100.0% (95%CI: 47.8-100.0%) and 86.7% (95%CI: 59.5-98.3%), respectively, and an accuracy of 90.0% (95%CI: 68.3-98.8%). Patients with log2 (C1/C0) > 0.044 showed worse progression-free survival than did those with log2 (C1/C0) ≤0.044 (median 2.0 versus 17.0 months; P = 0.092). Conclusions: Early changes in ctDNA that are detected via targeted sequencing could predict later radiologic responses in mCRC.