: Radical cystectomy performed via midline laparotomy is associated with substantial postoperative pain, frequently necessitating a high opioid consumption, which may impair immune function and delay recovery. The rectus sheath block (RSB) is widely used as part of multimodal analgesia to enhance postoperative pain control; however, the duration of analgesia is limited when using single-injection techniques. Dexamethasone has increasingly been used as a perineural adjuvant to prolong the effects of peripheral nerve blocks and enhance analgesia. This randomized controlled trial evaluated whether adding perineural dexamethasone to an RSB improves analgesic efficacy in patients undergoing a radical cystectomy. : Fifty-two adult patients scheduled for radical cystectomy were randomly assigned to receive an ultrasound-guided bilateral RSB with either 0.25% ropivacaine alone or 0.25% ropivacaine combined with 4 mg dexamethasone per side after skin closure. Postoperative pain was assessed using a numeric rating scale (NRS) at 3, 6, 12, 18, 24, and 48 h following surgery. Cumulative intravenous patient-controlled analgesia (IV-PCA) in terms of fentanyl consumption and the incidence of rebound pain-defined as an increase in the NRS from ≤3 to ≥7 within 24 h after the block administration-were also recorded. : The dexamethasone group exhibited significantly reduced cumulative fentanyl consumption. Pain scores were consistently lower in the dexamethasone group compared with the ropivacaine-only group at all time points except 3 h postoperatively. The incidence of rebound pain was also substantially lower in the dexamethasone group. : Perineural dexamethasone as an adjuvant to an RSB provides effective and prolonged analgesia, reduces opioid requirements, and lowers rebound pain incidence in patients undergoing a radical cystectomy.