Background: Single agent of panitumumab (Pmab) is expected to be well tolerated and to improve survival in first-line setting in patients (pts) who are not eligible for intensive chemotherapy, although the efficacy and safety of Pmab for chemotherapy-naïve frail or elderly Japanese pts with wild-type (wt) RAS unresectable colorectal cancer (CRC) have not been yet studied. Methods: We conducted a multi-center phase II study. Pts aged over 76 years, or over 65 who were considered unsuitable for intensive chemotherapy. Pmab 6 mg/kg was administered intravenously every 2 weeks. The primary endpoint was disease control rate (DCR), and secondary endpoints included progression-free survival (PFS), overall survival (OS), response rate (RR), time to treatment failure (TTF), and the incidence of grade 3 or 4 toxicities. Sample size was set to 36 with exact p-value of 0.05, a power of 0.90, the null of hypothesis of 45% and alternative hypothesis of 70% based on the Clopper-Pearson method. Results: A total 36 pts were enrolled in February 2017 to August 2018. Two pts were excluded; one was a lack of image examination at baseline, and the other was a lack of measurable lesion. The median age was 81 (67-88), with 29 pts (85%) being aged over 76 years. There were 33 (92%) pts with performance status (PS) 0/1, while two (6%) and one (3%) pts were PS 2/3, respectively. Twenty-eight pts (78%) had left-sided CRC, while eight pts had right-sided CRC. The RR was 50.0% (95%CI, 32.4-67.6) including three cases (8.8%) of complete response, and SD was 26.5%, yielding 76.5% of DCR (p < 0.001, 90% confidence interval [CI], 61.5-87.7). The RR with left sided tumor was 65% (95%CI, 44.3-82.8), while that pts with right-sided tumor was 0% (95%CI, 0.0-36.9)(p = 0.003). The major grade 3 or 4 nonhematologic toxicities were rash (n = 6, 17%), hypomagnesemia (n = 4, 11%), fatigue (n = 3, 8%), paronychia (n = 3, 6%), and hyponatremia (n = 3, 6%). The grade 3 hematologic toxicities was neutropenia (n = 1, 3%). Conclusions: Pmab monotherapy showed the favolable efficacy and feasibility in the frail or elderly pts with RAS wt, unresectable CRC. The survival analysis including OS, PFS and TTF is awaiting. Clinical trial information: UMIN000024528.