This study aimed to determine the molecular mechanism by which the oncogenic micoRNA-21 (miR-21) functions in bladder cancer. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis showed that the expression of miR-21 considerably increased in primary cancer tissue compared with that in the paired adjacent noncancerous tissue and that in normal bladder mucosa. Knockdown of miR-21 by using antisense oligonucleotide significantly suppressed the proliferation and migration of bladder cancer cells (J82 and RT112). Mechanism studies showed that downregulation of miR-21 resulted in cell cycle arrest at the G1 phase and upregulation of the tumor suppressor PTEN (phosphatase and tensin homologue) and p53 phosphorylation at Ser46. The p53 phosphorylation at Ser15 and the whole level of p53 acetylation remained unchanged in response to miR-21 knockdown. MicroRNA-21 regulates proliferation and migration of bladder cancer cells and cross talk with PTEN and p53 in bladder cancer.