Background <p>Anastomotic leakage remains one of the major complications after gastrectomy in patients with gastric cancer and may increase postoperative morbidity and delay recovery. Preoperative nutritional status is closely associated with anastomotic healing; however, the value of the Controlling Nutritional Status (CONUT) score for predicting clinically relevant anastomotic leakage after gastric cancer surgery requires further clarification. This study aimed to evaluate the predictive value of the preoperative CONUT score for clinically relevant anastomotic leakage after radical gastrectomy and to construct an interpretable risk prediction model and a three-tier risk stratification system based on predicted probability.</p> Methods <p>Clinical data from 283 patients who underwent radical gastrectomy from January 2021 to January 2025 were retrospectively analyzed. The CONUT score was calculated from serum albumin, total lymphocyte count, and total cholesterol. The primary endpoint was clinically relevant anastomotic leakage of grade II or higher within 30 postoperative days. Logistic regression was used to identify risk factors. Receiver operating characteristic (ROC) curve analysis was used to evaluate the predictive value of the CONUT score and its components. Internal validation was performed using bootstrap resampling with 1000 repetitions, and exploratory temporal validation was conducted by using patients treated from January 2021 to December 2023 as the modeling cohort and those treated from January 2024 to January 2025 as the temporal validation cohort.</p> Results <p>Among the 283 patients, 39 (13.8%) developed clinically relevant anastomotic leakage. Multivariable Logistic regression showed that CONUT score (odds ratio [OR] = 1.425, 95% confidence interval [CI]: 1.168–1.739, <i>P</i> &lt; 0.001), diabetes mellitus (OR = 2.486, 95% CI: 1.052–5.876, <i>P</i> = 0.038), total gastrectomy (OR = 2.318, 95% CI: 1.028–5.228, <i>P</i> = 0.043), operative time (OR = 1.008, 95% CI: 1.002–1.014, <i>P</i> = 0.012), and intraoperative blood loss (OR = 1.004, 95% CI: 1.001–1.007, <i>P</i> = 0.018) were independent risk factors for anastomotic leakage. The area under the curve (AUC) of the CONUT score for predicting anastomotic leakage was 0.782 (95% CI: 0.716–0.848). When a CONUT score ≥ 3 was used as a sensitivity-prioritized screening threshold, the sensitivity, specificity, and negative predictive value were 74.4%, 62.3%, and 93.8%, respectively; when a score ≥ 4 was used as a higher-specificity alternative threshold, the sensitivity and specificity were 59.0% and 78.3%, respectively. Compared with serum albumin alone, the AUC of the CONUT score showed a modest increase of 0.058 (DeLong test <i>P</i> = 0.042). The risk prediction model based on five independent risk factors had an apparent concordance index (C-index) of 0.838 (95% CI: 0.779–0.897), a bootstrap-corrected C-index of 0.826, and a C-index of 0.803 (95% CI: 0.701–0.905) in the temporal validation cohort. After three-tier risk stratification based on predicted probability, the incidence of anastomotic leakage increased stepwise in the low-, intermediate-, and high-risk groups (<i>χ²</i> = 56.925, <i>P</i> &lt; 0.001).</p> Conclusions <p>The preoperative CONUT score was associated with an increased risk of clinically relevant anastomotic leakage after gastric cancer surgery. A risk prediction model incorporating CONUT score, diabetes mellitus, total gastrectomy, operative time, and intraoperative blood loss may provide a reference for perioperative risk assessment, although multicenter external validation is still required before broader application.</p>