Preoperative nutritional status is an important prognostic factor in gastric cancer patients. This study will evaluate whether preoperative oral dysfunction is associated with prognostic nutrition index (PNI). This case-control study analyzed 95 patients who underwent oral function management. We assessed the following parameters: body mass index, stage of gastric cancer, C-reactive protein, total lymphocyte count, albumin, and prognostic nutritional index. The patients were divided into groups with prognostic nutritional indexes <45 and >45. Logistic regression analysis was used to assess the association between the measurements of oral function and the prognostic nutritional index. Univariate analysis of factors associated with decreased oral function and prognostic nutritional index showed significant differences between the two groups in C-reactive protein, neutrophils, and tongue pressure (p<0.01). However, oral hygiene, oral dryness, occlusal force, tongue–lip motor function, masticatory function, and swallowing function were not significantly different. Multivariate analysis showed that C-reactive protein (odds ratio: 0.12, 95% confidence interval: 0.30–0.45, p<0.01) and tongue pressure (odds ratio: 3.62, 95% confidence interval: 1.04–12.60, p<0.05) were independent risk factors for oral hypofunction. Oral function decreased in perioperative patients with gastric cancer, and decreased tongue pressure is associated with a decreased prognostic nutritional index.