Background: Cyclin-dependent kinase (CDK) 10, is reported to play an essential role in the progression from the G2 to M phase of the cell cycle. Recently, reduced expression of CDK10 has been observed in several cancerous human tissue, suggesting that CDK10 is a tumor suppressor gene. However, data on its expression pattern and clinical relevance in gastric cancer are not sufficient. Therefore, this study aims to investigate CDK10 expression and its prognostic significance in primary gastric adenocarcinoma. Methodology/Principal Findings: The expression level of CDK10 was analyzed using qRT-PCR, western blotting, and immunohistochemistry on tissue samples from 189 post-resection gastric cancer patients. The expression of CDK10 mRNA was reduced in tumor tissue samples compared with matched adjacent non-tumor tissue samples (P=0.013); this finding was confirmed by western blot analysis (P=0.016). Immunohistochemistry data indicated that CDK10 expression was significantly decreased in 92 of 189 (48.7%) gastric cancer cases. Kaplan-Meier survival curves revealed that decreased expression of CDK10 was strongly associated with a poor prognosis in gastric cancer patients (P<0.001). Multivariate Cox analysis identified CDK10 expression as an independent prognostic factor for overall survival (P=0.011). Conclusions/Significance: Our data suggest that reduced CDK10 expression independently predicts a poor prognosis in patients with gastric cancer. CDK10 can may serve as a valuable prognostic marker and a potential target for gene therapy.