Cover Story：Elongation factor 4 (EF4) is a non-conventional elongation factor which regulates protein synthesis in mitochondria. In this study, we explored its function in bladder urothelial carcinoma. By analyzing the expression of EF4 in bladder urothelial carcinoma and adjacent normal tissues, we found that EF4 was aberrantly elevated in multiple cohorts of bladder cancer patients. Notably, the upregulation of EF4 was positively associated with tumor progression. By manipulating EF4 expression in HTB-9 and T-24 bladder cancer cells, the effects of upregulated EF4 was investigated. Knockdown of EF4 suppressed the proliferation and colony formation in bladder cancer cells; the ability of cells to migrate in vitro was also retarded. Knockdown of EF4 down-regulated the expression of mitochondrial DNA-encoded subunits of electron transfer chain complexes, and resulted in the dysfunction of mitochondrial oxidative phosphorylation. These results define a tumor-supportive role for EF4 by maintaining the protein synthesis within the mitochondria, which may serve as a potential therapeutic target in bladder urothelial carcinoma.