Platycodin D (PD) is a major constituent of Platycodon grandiflorum and has multiple functions in disease control. This study focused on the function of PD in bladder cancer cell behaviors and the molecules involved. First, we administered PD to the bladder cancer cell lines T24 and 5637 and the human uroepithelial cell line SV-HUC-1. Cell viability and growth were evaluated using MTT, EdU, and colony formation assays, and cell apoptosis was determined using Hoechst 33342 staining and flow cytometry. The microRNAs (miRNAs) showing differential expression in cells before and after PD treatment were screened. Moreover, we altered the expression of miR-129-5p and PABPC1 to identify their functions in bladder cancer progression. We found that PD specifically inhibited the proliferation and promoted the apoptosis of bladder cancer cells; miR-129-5p was found to be partially responsible for the cancer-inhibiting properties of PD. PABPC1, a direct target of miR-129-5p, was abundantly expressed in T24 and 5637 cell lines and promoted cell proliferation and suppressed cell apoptosis. In addition, PABPC1 promoted the phosphorylation of PI3K and AKT in bladder cancer cells. Altogether, PD had a concentration-dependent suppressive effect on bladder cancer cell growth and was involved in the upregulation of miR-129-5p and the subsequent inhibition of PABPC1 and inactivation of PI3K/AKT signaling.