Purpose . To evaluate the predictive potency of a novel index combining the pan‐immune‐inflammatory index and hemoglobin levels (PIV/Hb) for the prevalence of radiation‐induced trismus (RIT) in patients with locally advanced nasopharyngeal cancer (LA‐NPC) receiving concurrent chemoradiotherapy (CCRT). Methods . Data from 228 LA‐NPC patients were retrospectively examined. Maximum mouth openings (MMO) were measured to confirm the presence of RIT, defined as MMOs ≤35 mm. Complete blood test results from the first day of CCRT were used to calculate PIV/Hb levels. A potential relationship between pretreatment PIV/Hb and the RIT status was evaluated using receiver operating characteristic (ROC) curve analysis. Results . Post‐CCRT RIT was diagnosed in 20.2% of the patients. The ROC curve analysis determined 68.4 g/dL as the ideal PIV/Hb cutoff that effectively divided patients into two distinct groups (area under the curve: 94.7%; specificity: 86.4%; sensitivity: 87.4%). RIT was significantly more prevalent in the PIV/Hb > 68 group than in the PIV/Hb < 68 group (58.8% vs. 3.8%; P < 0.001). Multivariate logistic regression analysis showed that a pre‐CCRT PIV > 68 was independently associated with significantly higher rates of RIT. Conclusion . Higher pretreatment levels of the novel PIV/Hb index predict increased RIT rates following definitive CCRT for LA‐NPCs.