Aims: Our study assessed the efficacy and safety of sintilimab-based regimens for real-world treatment of advanced gastric and gastroesophageal junction adenocarcinoma (G/GEJAC). Materials and Methods: Cases of advanced nonresectable G/GEJAC treated with sintilimab-based regimens in the Department of Gastroenterology of Shanxi Provincial Cancer Hospital between December 2018 and September 2020 were retrospectively examined. Endpoints included median progression-free survival (mPFS), median overall survival (mOS), disease control rate (DCR), objective response rate (ORR), and adverse events (AEs). Univariate and multivariate analyses were conducted to determine the effect of stratification factors on efficacy. Results: Among the 37 included patients, mPFS and mOS were 4.27 and 7.3 months, respectively. Efficacy was evaluated at least once in 32 of 37 patients. The ORR and DCR were 12.5% and 65.63%, respectively. Among four patients with mismatch repair deficiency/microsatellite instability-high (dMMR/MSI-H) lesions, two achieved partial remission, and two displayed stable disease, resulting in a DCR of 100%. The most observed AEs included leukopenia, neutropenia, thrombocytopenia, nausea, and skin rash. mPFS was 4.90 months in patients who received sintilimab in the first- or second-line setting, versus 3.00 months in other patients. A significant survival difference was found between these groups in univariate and multivariate analyses. Conclusions: The application of sintilimab-based regimens achieved good disease control and tolerability for the real-world treatment of advanced G/GEJAC. The treatment was more effective when administered in the first- or second-line setting. Patients with dMMR/MSI-H lesions may also benefit from sintilimab-based regimens.