Background: Perioperative treatment is very important to improve long-term survival for gastric/gastroesophageal junction cancer(GC/GEJC), The RESOLVE study showed that perioperative-SOX have a clinically meaningful improvement in patients with locally advanced GC/GEJC who had D2 gastrectomy. Adding PD-1 inhibitor to the chemotherapy have shown significant clinical benefits in first-line treatment of GC/GEJC. This trial was designed to assess the feasibility and efficacy of this combination in perioperative treatment of resectable locally advanced GC/GEJC. Methods: This is a prospective, single-arm, single-center phase II study. Patients of histopathology confirmed locally advanced GC/GEJC, with clinical stage II-IV A per AJCC8^th, ECOG PS 0-1, were enrolled and treated with 3 cycles of preoperative sintilimab (200mg, iv, d1) and SOX (oxaliplatin 130mg/m², iv, d1 and S-1 40-60mg, po, bid, d1-14) every 3 weeks, and 12 months of postoperative sintilimab (200mg, iv, d1) and S-1 (40-60mg, po, bid, d1-14) every 3 weeks. The primary endpoint was 2-year DFS rate. The second endpoints were pCR, MPR, R0 resection rate, safety and 3-year OS. Results: As of June 2021, 21 patients were enrolled, with median age 56 years (range 31-72 years), males 10 (47.6%), cT2/3/4a/4b 2(9.5%)/0/16(76.2%)/3(14.3%), cN1/N2 7(33.3%)/14(66.7%) and GC/GEJC 14(66.7%)/7(33.3%). All 21 patients had completed gastrectomy. 7 patients (33.3%) achieved pathological complete response (pCR), 8 patients (38.1%) had major response (TRG 0-1) and 21 patients (100%) achieved R0 resection. Common TRAEs were anemia (38.1%), lymphopenia (28.6%), neutropenia (19.0%), leukopenia (14.3%), ALT increase (9.5%), AST increase (4.8%), thrombocytopenia (4.8%). Grade 3 or more TRAEs included anemia (9.5%). There were no severe complications and death related to the operation. The median postoperative hospital stay was 10 days (range 7-41 days). Conclusions: Adding sintilimab to the chemotherapy resulted in an encouraging pCR and MPR as perioperative treatment for resectable locally advanced GC/GEJC, and safety was manageable. A Phase II randomized study is ongoing. Clinical trial information: ChiCTR2100043572.