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Background and Aims Precise needle tip localisation is critical for safe and effective peripheral nerve blocks. Ultrasound, nerve stimulation, and injection pressure monitoring assist in this process, but may not always ensure optimal placement. Electrical impedance (EI), sensitive to tissue composition and water content, has emerged as a potential adjunctive marker. This study aimed to characterise EI variations during ultrasound-guided sciatic nerve block (SNB) and to evaluate their utility in identifying needle position relative to the paraneural sheath.Methods In this prospective observational study, 45 adult patients undergoing lower-limb surgery under SNB were enrolled. EI measurements were obtained at four predefined needle positions: intramuscular, outside the paraneural sheath, within the sheath, and post-injection. Blinded video review of local anaesthetic spread by three independent anaesthetists classified patients into two groups: confirmed within-sheath injection and extra-sheath injection. Receiver operating characteristic (ROC) analysis assessed the diagnostic performance of EI changes.Results Of the 45 patients, 31 had confirmed within-sheath injections, and 14 had extra-sheath injections. In the confirmed group, EI significantly increased approaching the sheath (mean difference +1.568, p = 0.0306) and decreased upon sheath entry (mean difference −1.445, p = 0.0010). No significant EI changes were observed in the extra-sheath group. ROC analysis demonstrated high diagnostic accuracy (AUC 0.890 for absolute EI change; AUC 0.860 for relative change).Conclusions Monitoring electrical impedance provides a valuable adjunct for needle tip verification during ultrasound-guided sciatic nerve block. When combined with ultrasound, it may improve block success rates and help reduce complications, enhancing both the safety and efficacy of regional anaesthesia in clinical practice.