Heterogeneous nuclear ribonucleoprotein K (hnRNP K) is one of the major pre-mRNA-binding proteins, that is involved in translational modifications. In our previous studies, we found that hnRNP K is associated with human gastric cancer. The protein levels of hnRNP K were detected in cell lines and tissue microarrays. The correlation between hnRNP K expression and patient survival rate was evaluated by Kaplan-Meier survival analysis. In addition, we also detected hnRNP K expression in preoperative and postoperative serum samples from patients with gastric cancer, and serum samples from healthy volunteers. We found that hnRNP K was overexpressed in the gastric cancer cell lines. The levels of hnRNP K were significantly elevated in the gastric cancer tissues compared with that noted in the tumor-adjacent gastric mucosal and normal gastric mucosal sampes, and hnRNP K expression was found to correlate with tumor stage and lymph node metastasis. However, the level of serum hnRNP K did not differ significantly between gastric cancer patients and healthy volunteers. We also found that patients whose tumors showed elevated expression of hnRNP K had poor survival. The present study suggests that hnRNP K is a promising tissue biomarker for diagnosing gastric cancer and is a prognostic indicator for patients with gastric cancer.