The tumor microenvironment (TME) has become an important area of investigation with respect to improving prognosis in malignancies. Here we evaluated TME prognostic risk in small intestinal adenocarcinomas based on histologic assessment of tumor budding at the peritumoral-invasive front (pTB) and stromal tumor-infiltrating lymphocytes (sTILs). pTB and sTILs were analyzed in 230 surgically resected small intestinal adenocarcinomas, as recommended by the International Tumor Budding Consensus Conference (ITBCC) and the International TILs Working Group (ITWG). On the basis of high levels of pTB count (≥10) and sTIL density (≥20%), we combined pTB and sTIL to produce a collective TME-based prognostic risk index: low-risk (pTBLow/sTILHigh; n=39, 17.0%), intermediate-risk (pTBLow/sTILLow or pTBHigh/sTILHigh; n=99, 43.0%), and high-risk groups (pTBHigh/sTILLow; n=92, 40.0%). TME risk index provided better prognostic stratification than the individual pTB and sTIL (14.9 vs. 6.7 vs. 10.3). Tumors with higher TME prognostic risk were associated with an infiltrative growth pattern and nonintestinal immunophenotype (both P=0.001), pancreatic invasion (P=0.010), lymphovascular (P<0.001) or perineural invasion (P=0.006), higher T-category (P<0.001), N-category (P=0.004), and stage grouping (P=0.002), and KRAS mutation (P=0.008). In multivariate analysis, higher TME prognostic risk index (P<0.001), distal tumor location and nonintestinal immunophenotype (both P=0.001), higher N-category (P<0.001), and microsatellite stable (P=0.015) were worse-independent prognosticators. TME prognostic risk index consistently stratified patient survival regardless of tumor location (P<0.001 in proximal; P=0.002 in distal), stages (P<0.001 in lower stages I to II; P=0.028 in stage III), and DNA mismatch repair gene status (P<0.001 in microsatellite stable; P=0.001 in microsatellite instability). TME risk index is a powerful prognostic predictor for risk stratification of patients with small intestinal adenocarcinoma.