Background: Early tumor shrinkage (ETS) and the depth of response (DpR) are early on-treatment surrogate markers for survival in patients with metastatic colorectal cancer (mCRC) receiving anti-epidermal growth factor receptor (EGFR) monoclonal antibody. However, the utility of ETS and DpR for BRAF V600E mutant (MT) mCRC which has a poor prognosis remains unclear. The aim of this study is to evaluate the association between ETS and DpR and clinical outcomes in BRAF V600E MT mCRC. Methods: mCRC patients who were diagnosed BRAF V600E MT and treated with 1^st line chemotherapy from June 2011 to March 2023 at single cancer institute were enrolled. We analyzed the association between ETS and DpR and clinical outcome in patients who had at least one target lesion. Subgroup analysis of clinical factors related to progression free survival (PFS) and overall survival (OS) was performed using multivariate analysis. ETS was defined as the relative change in the sum of the longest diameters of RECIST target lesions at first follow-up CT scan compared to baseline and DpR was defined as the relative change in the sum of the longest diameters of RECIST target lesions at the nadir in the absence of new lesions or progression of non-target lesions compared to baseline. Results: In total, 54 patients of BRAF V600E MT mCRC had at least one target lesion. In total of StageI-IV CRC patients, the incidence of BRAF V600E MT was 6.3% (233/3705). Patients with ETS ≥ 20% and DpR ≥ 25% (median value) were 24 (44.4%) and 27 (50%) respectively. PFS was 7.5 months (95% confidence interval (CI), 5.0-9.9) and OS was 17.1 months (95% CI, 11.7-20.2). Patients with ETS ≥ 20% had longer PFS and tended to have longer OS than those with non-ETS, with a median PFS of 9.8 months vs. 4.8 months (P _Log-rank = 0.048, hazard ratio (HR), 0.55; 95% CI, 0.30-1.00), and a median OS of 22.6 months vs. 11.6 months (P _Log-rank = 0.18, HR, 0.67; 95% CI, 0.37-1.21). Patients with DpR ≥ 25% had also longer PFS and OS than those with non-DpR, with a median PFS of 11.0 vs. 4.3 months (P _Log-rank< 0.01, HR, 0.36; 95% CI, 0.20-0.66) and a median OS of 22.6 vs. 10.1 months (P _Log-rank = 0.047, HR, 0.55; 95% CI, 0.31-1.00). Median time to DpR was 3.05 months. In multivariate analysis, DpR was significantly associated with both longer PFS (HR: 0.27, 95% CI: 0.14–0.55, P < 0.01) and OS (HR: 0.52, 95% CI: 0.29–0.96, P = 0.04). Conclusions: ETS and DpR may be early surrogate markers for clinical outcome in BRAF V600E MT mCRC who were treated with 1^st line chemotherapy.