Homeobox (HOX) genes encode proteins that function as transcription factors during embryogenesis and tumorigenesis. We have previously reported upregulation of HOXC10 in gastric cancer (GC) tissues using cDNA microarray analysis. Though the functional role of HOXC10 in GC has been briefly reported, its specific mechanism is not fully understood. We analyzed the expression of HOXC10 in GC tissues, as well as its correlation with the survival outcome. By in vitro and in vivo assays, we further investigated the role of HOXC10 on cell cycle control and proliferation. Finally, we screened potential downstream targets of HOXC10 by cDNA microarray and explored the role of HOXC10 in p21 transcriptional repression through a dual luciferase reporter and chromatin immunoprecipitation. We illustrated the upregulation of HOXC10 in GC tissues and high HOXC10 expression related to poor survival outcome. Multivariable COX regression analysis showed that HOXC10 was an independent predictor of survival (HR=1.863; 95% CI: 1.076-3.225). Functionally, HOXC10 could promote GC cell proliferation and tumor growth in nude mice. Overexpression of HOXC10 accelerated G1/S cell cycle transition, whereas knocking down HOXC10 induced cell cycle arrest at the G1 phase. Critical factors of G1/S cell cycle transition including p21, CDK2, and c-Myc, were regulated by HOXC10. Importantly, an inverse correlation between p21 and HOXC10 expression in GC cell lines and tissues was observed. HOXC10 could directly bind to the promoter region of p21 and repress its transcriptional activity. Collectively, we identified HOXC10 as a predictor of poor prognosis in GC patients, and a novel transcriptional regulator of p21 in the G1/S cell cycle transition.