Background: Perioperative treatments have significantly improved survival in patients with resectable gastric cancer, increasing 5-year overall survival from 23% with surgery alone to 45% with FLOT, Although FLOT has been recognized as the first choice for neoadjuvant chemotherapy in gastric or GEJ adenocarcinoma, its efficacy needs to be improved. Sintilimab, a fully human IgG4 monoclonal antibody that binds to programmed cell death receptor-1 (PD-1), has shown remarkable clinical efficacy in various cancers. We aimed to assess the activity and safety profile of the combination of FLOT and sintilimab for neoadjuvant treatment of gastric or GEJ adenocarcinoma. Methods: In this ongoing, single-arm, phase II study, we recruited patients from Henan Cancer Hospital in China with histopathologically diagnosed resectable gastric or GEJ adenocarcinoma who had clinical T3/N+ or higher stage. Patients were given 4 cycles of FLOT (docetaxel 50 mg/m2, oxaliplatin 80 mg/m2, leucovorin 200 mg/m2, fluorouracil 2600 mg/m2, 24-h infusion on day 1, q2w) in combination with 3 cycles of sintilimab (200mg, iv, d1, q3w), followed by D2 surgery and 4 postoperative cycles of FLOT. The primary endpoint was pathological complete response (pCR). The secondary endpoints included major pathological remission (MPR) and R0 resection rate and adverse events . Results: A total of 20 patients were enrolled in the study between Aug 10 2019 and Sep 15 2020. One patient refused surgery, one person's disease progressed. Two patients have not yet completed neoadjuvant treatment . 16 pts who experienced D2 resection, 10 (62.5%) achieved major pathologic response (MPR), including 3 (18.8%) with a pathologic complete response (pCR) in primary tumor. The R0 resection rate was up to 93.8%, The grade 3 or 4 treatment-related adverse events (TRAE) included lymphopenia(25%), anaemia (20%),fatigue (20%),leucopenia (15%), neutropenia (5%), diarrhea(5%), Alanine aminotransferase increased(5%),There was no surgical delays or unexpected surgical complications related to drug toxicity. Conclusions: Neoadjuvant combination of sintilimab and FOLT is a safe and efficacious treatment option for patients with gastric or GEJ adenocarcinoma, 18.8% pCR rate and 62.5%MPR rate is encouraging. Our clinical study is still enrolling, and the survival effects are under follow up. Clinical trial information: NCT04341857