The tumor immune microenvironment can provide prognostic and therapeutic information. We aimed to develop noninvasive imaging biomarkers from computed tomography (CT) for comprehensive evaluation of immune context, and investigate their associations with prognosis and immunotherapy response in gastric cancer (GC).This study involved 2,600 GC patients of nine independent cohorts. We developed and validated two CT imaging biomarkers [lymphoid radiomics score (LRS) and myeloid radiomics score (MRS)] for evaluating the immunohistochemistry (IHC)-derived lymphoid and myeloid immune context respectively, and then integrated them into a combined imaging biomarker [LRS/MRS: low(-) or high(+)] with four radiomics immune subtypes: 1(-/-), 2(+/-), 3(-/+), and 4(+/+). We further evaluated the imaging biomarkers' predictive values on prognosis and immunotherapy response.The developed imaging biomarkers (LRS and MRS) had a high accuracy in predicting lymphoid (AUC range: 0.765-0.773) and myeloid (AUC range: 0.736-0.750) immune context. Furthermore, same as IHC-derived immune context, two imaging biomarkers (HR range: 0.240-0.761 for LRS; 1.301-4.012 for MRS) and the combined biomarker were independent predictors for disease-free and overall survival in the training and all validation cohorts (all P<0.05). In addition, patient with high LRS or low MRS may benefit more from immunotherapy (P<0.001). Furthermore, a highly heterogeneous outcome on objective response rate was observed in four imaging subtypes: 1(-/-) with 27.3%, 2(+/-) with 53.3%, 3(-/+) with 10.2%, and 4(+/+) with 30.0% (P<0.0001).The noninvasive imaging biomarkers could accurately evaluate the immune context, and provide information regarding prognosis and immunotherapy for GC.None.