JCOG0603 demonstrated improved disease-free survival (DFS) with adjuvant mFOLFOX6 after hepatectomy compared with hepatectomy alone in colorectal liver-only metastasis (CRLM), but the overall survival (OS) data were immature. Here, we report OS after long-term follow-up. Eligible patients with colorectal adenocarcinoma and an unlimited number of liver metastases were randomized to adjuvant mFOLFOX6 or hepatectomy alone. DFS was the primary endpoint, and OS was a secondary endpoint. Between March 2007 and January 2019, 151 and 149 patients were randomized to adjuvant chemotherapy and hepatectomy alone, respectively. At a median follow-up of 7.7 years for disease-free surviving patients, 54 (35.8%) and 51 (34.2%) patients had died in the respective arms, (hazard ratio [HR], 1.07 [95% CI, 0.73 to 1.57]). Five-year OS was 73.4% (95% CI, 65.5 to 79.7) and 80.1% (95% CI, 72.6 to 85.7) and 7-year OS was 69.4% (95% CI, 61.2 to 76.2) and 72.4% (95% CI, 64.2 to 79.1), respectively. One patient in the adjuvant chemotherapy arm possibly died of protocol treatment-related toxicity, and one in the hepatectomy-alone arm died of post-protocol treatment complications. Five-year DFS was respectively 49.7% (95% CI, 41.5 to 57.3) and 40.5% (95% CI, 32.5 to 48.3) in the adjuvant chemotherapy and hepatectomy-alone arms (HR, 0.72 [95% CI, 0.54 to 0.97]). Long-term OS did not differ with adjuvant mFOLFOX6 compared with hepatectomy alone in resectable CRLM. Adjuvant mFOLFOX6 may delay recurrence but did not improve long-term survival.