In 2–5% of patients with colorectal cancer (CRC), human epidermal growth factor 2 (HER2) is amplified or overexpressed. Despite prior evidence that anti-HER2 therapy confers clinical benefit (CB) in one-third of these patients, it is not approved for this indication in Europe. In the Drug Rediscovery Protocol (DRUP), patients are treated with off-label drugs based on their molecular profile. Here, we present the results of the cohort ‘trastuzumab/pertuzumab for treatment-refractory patients with RAS/BRAF-wild-type HER2amplified metastatic CRC (HER2+mCRC)’. Patients with progressive treatment-refractory RAS/BRAF-wild-type HER2+mCRC with measurable disease were included for trastuzumab plus pertuzumab treatment. Primary endpoints of DRUP are CB (defined as confirmed objective response (OR) or stable disease (SD) ≥ 16 weeks) and safety. Patients were enrolled using a Simon-like 2-stage model, with 8 patients in stage 1 and 24 patients in stage 2 if at least 1/8 patients had CB. To identify biomarkers for response, whole genome sequencing (WGS) was performed on pre-treatment biopsies. CB was observed in 11/24 evaluable patients (46%) with HER2+mCRC, seven patients achieved an OR (29%). Median duration of response was 8.4 months. Patients had undergone a median of 3 prior treatment lines. Median progression-free survival and overall survival were 4.3 months (95% CI 1.9–10.3) and 8.2 months (95% CI 7.2–14.7), respectively. No unexpected toxicities were observed. WGS provided potential explanations for resistance in 3/10 patients without CB, for whom WGS was available. The results of this study confirm a clinically significant benefit of trastuzumab plus pertuzumab treatment in patients with HER2+mCRC. •In 2-5% of patients with colorectal cancer HER2 is amplified or overexpressed.•Trastuzumab/pertuzumab treatment confers clinical benefit in almost half of HER2+mCRC patients.•Treatment with trastuzumab plus pertuzumab was well tolerated.•Whole genome sequencing data may reveal potential resistance mechanisms to trastuzumab/pertuzumab treatment.