Bladder cancer is one of the most common types of urothelial carcinoma with a rising incidence rate worldwide. Circular RNAs (circRNAs) are involved in the development of numerous cancers, including bladder cancer. We aimed to uncover the role and associated mechanism of circMYLK in bladder cancer. The expression levels of circMYLK, miRNA-34a (miR-34a) and Cyclin D3 (CCND3) mRNA were investigated using real-time quantitative polymerase chain reaction. The protein level of CCND3 was investigated using western blot. In functional assays, flow cytometry assays were utilized for cell cycle analysis and cell apoptosis analysis. Transwell assays were used for cell migration and invasion analysis. Caspase-3 activity was examined to monitor cell apoptosis. The putative relationship between miR-34a and circMYLK or CCND3 was validated by dual-luciferase reporter assay and RNA immunoprecipitation assay. CircMYLK was highly expressed in bladder cancer tissues and cells. CircMYLK downregulation inhibited bladder cancer cell migration and invasion, and promoted cancer cell apoptosis and cell cycle arrest. MiR-34a, a target of circMYLK, was downregulated in bladder cancer tissues and cells. MiR-34a inhibition reversed the effects of circMYLK downregulation and then recovered bladder cell malignant behaviors. Further analysis showed that CCND3 was a downstream target of miR-34a, and CCND3 was upregulated in bladder cancer tissues and cells. MiR-34a overexpression blocked bladder cancer cell migration and invasion, and induced cell apoptosis and cycle arrest, while these effects were abolished by CCND3 overexpression. CircMYLK contributed to the malignant development of bladder cancer cells partly through the miR-34a/CCND3 regulatory network, showing the significance of circMYLK in bladder cancer pathogenesis.