In this single-arm, phase I study (NCT04117087), we evaluated mKRAS-VAX, a pooled mutant KRAS (mKRAS) peptide vaccine targeting six KRAS mutations with nivolumab and ipilimumab in 13 patients with pretreated metastatic MMRp/MSS CRC. mKRAS-VAX elicited an increase in tumor-specific mKRAS-reactive T-cells in 8/12 biomarker-evaluable patients (75%) by direct ex vivo IFNγ ELISpot and in 12 patients (100%) following in vitro expansion. Our findings support further development of mKRAS vaccines with ICIs for advanced MMRp/MSS CRC.