Background: PD-1 blockade has been recommended as first-line therapy for non-resectable or metastatic mismatch repair-deficient/microsatellite instability-high (dMMR/MSI-H) colorectal cancer (CRC). However, the safety and efficacy of neoadjuvant immunotherapy of PD-1 blockade for resectable dMMR/MSI-H CRC remains unclear. Methods: From June 2020 and March 2021, eight resectable dMMR/MSI-H CRC patients treated in the 6th Affiliated Hospital of Sun Yat-sen University (Guangzhou, China) were enrolled. All patients accepted 6 doses (200mg/dose, every 3 weeks) of Sintilimab injection (Innovent, LTD) before radical laparoscopic resections. The clinical and pathological data of the 8 patients was analyzed retrospectively. Results: All patients were confirmed dMMR by immunohistochemistry (IHC). However, 87.5% (7/8) of the patients were confirmed MSI-H and 12.5% (1/8) was confirmed microsatellite stable (MSS) by NGS. After 6 doses of neoadjuvant anti-PD-1 therapy, 87.5% (7/8) of the patients achieved pathological complete response (pCR), and the 7 patients were confirmed both dMMR and MSI-H. The rest 12.5% (1/8) who achieved major pathological response (mPR), with residual tumor <1%, was dMMR but MSS. None of the patients had immunotherapy-related adverse events (irAE) of grade 3 or above (CTCAE; version 5.0). 75.0% (6/8) of the patients had grade 1/2 irAE. No operational mortality or complications were found within 30 days after surgery. Conclusions: Single-agent neoadjuvant immunotherapy of PD-1 antibody was safe and effective in resectable dMMR/MSI-H CRC. Double confirmation of both dMMR and MSI-H status was necessary before PD-1 blockade therapy in dMMR/MSI-H CRC patients.