Background: We examined the frequency, tumor characteristics, and prognostic impact of HER2 protein expression in stage II and III colon cancer after curative resection. Methods: Paraffin-embedded tumors from consecutive primary stage II and III colon cancer patients were analyzed for HER2 protein expression by immunohistochemistry between April, 2013 and May, 2020. HER2 determination of immunohistochemistry scores (0/1+/2+/3+) was according to HERACLES diagnostic criteria. Results: A total of 2088 stage II and III colon cancer patients were included (53.8% stage II, 46.3% stage III). HER2 scored positive (3+) was detected in 48(2.3%) tumors, and was correlated with younger age (P < 0.001), well/moderate differentiation (P = 0.026), proficient mismatch repair (pMMR) (P = 0.045) and KRAS wild-type (P < 0.001). HER2 scored positive (3+) was not significantly associated with disease-free survival (DFS) compared with HER2 scored negative (0/1+), neither in stage III patients (multivariable HR, 0.86; 95CI, 0.38 to 1.94; P = 0.717), nor in stage II patients (multivariable HR, 1.68; 95CI, 0.74 to 3.84; P = 0.218). In a separate analysis involving stage II patients without any high-risk factor (n = 741), those with HER2 scored positive (3+) tumors (n = 16) showed significantly reduced DFS (multivariable HR, 2.91; 95CI, 1.04 to 8.81; P = 0.041) compared with patients with HER2 scored negative (0/1+) tumors, independent of sex, age and MMR status. Conclusions: HER2 scored positive (3+) was independently associated with poor DFS in stage II colon cancer patients without high-risk factors. HER2 expression determination may help to judge the prognosis of those patients and guide adjuvant chemotherapy.