Interstitial cystitis (IC) is a chronic inflammatory condition of the urinary bladder with a strong autoimmune component. Currently, the major challenge in IC treatment is the development of effective therapies. RDP58 is a novel d-amino acid decapeptide with potent immunosuppressive activity. In this study, we investigated whether RDP58 was effective as an intravesical agent for treating bladder autoimmune inflammation in a transgenic mouse model (URO–OVA mice). URO-OVA mice were adoptively transferred with syngeneic activated splenocytes of OT-I mice transgenic for the OVA-specific CD8⁺ TCR for cystitis induction and treated intravesically with RDP58 at days 0 and 3. Compared with controls, the RDP58-treated bladders showed markedly reduced histopathology and expressions of mRNAs and proteins of TNF-α, NGF and substance P. To determine whether the inhibition of bladder inflammation by RDP58 was due to the interference with effector T cells, we treated the cells with RDP58 in vitro. Cells treated with RDP58 showed reduced production of TNF-α and IFN-γ as well as apoptotic death. Collectively, these results indicate that RDP58 is effective for treating T cell-mediated experimental autoimmune cystitis and may serve as a useful intravesical agent for the treatment of autoimmune-associated bladder inflammation such as IC.