NPM1 plays an important role in the occurrence and development of leukemia and various solid tumors. This study aimed to investigate the expression of NPM1 in gastric cancer (GC) and adjacent normal tissues, study the relationship between NPM1 expression and clinicopathological characteristics in GC patients, and explore the impact of NPM1 expression on the diagnosis and prognosis of GC. We used tissue microarray immunohistochemical analysis to examine the expression level of NPM1 in GC and adjacent tissues and analyzed the relationship between NPM1 expression, clinicopathological factors, and GC prognosis. Prognostic values of NPM1 mRNA were also investigated using an online database. qRT-PCR was used to detect the expression of NPM1 mRNA in cancer and adjacent tissues. According to microarray immunohistochemical analysis and qRT-PCR results, NPM1 had a high expression in all adjacent normal tissues. Microarray immunohistochemical analyses demonstrated that the NPM1 was lowly expressed in 75.5% of GC tissues but highly expressed in 24.5% of GC tissues. qRT-PCR results showed NPM1 mRNA low expression in most GC tissues. NPM1 high expression group was associated with a better overall survival rate and disease-free survival rate than the NPM1 low expression group (p<0.01). This result is consistent with that of the online database. The receiver operating characteristics curve showed that NPM1 was valuable in the diagnosis of GC. The assessment of NPM1 expression in GC samples may represent a useful tool for GC diagnosis and prognosis assessment.