Objectives: To compare the postoperative analgesia efficacy of dexamethasone added to low-dose high-volume bupivacaine in ultrasound-guided supraclavicular brachial plexus block for adult patients who underwent upper limb orthopedic surgeries at Tibebe Ghion Specialized Hospital, Bahir Dar, Ethiopia. Methods: An observational prospective cohort study was conducted from 1 September 2021 to 30 January 2022. Using a systematic random sampling technique, 56 patients (equal groups of 28 patients) aged 18–60 years scheduled for elective upper limb orthopedic surgeries under supraclavicular block were recruited. According to the discretion of anesthetists’ management plan of supraclavicular block, those patients who received 38 mL of 0.25% bupivacaine with 2 mL (8 mg) dexamethasone as the case group (DB) while those patients who received 40 mL of 0.25% bupivacaine alone as the cohort group (B). Time to first analgesic request, onset of sensory and motor, and motor block duration were analyzed with Student’s t-test whereas pain severity and analgesic consumption were assessed using the Mann–Whitney U test. Categorical variables were analyzed using chi-square test. p-value less than 0.05 considered statistically significant. Results: Postoperative analgesia had significantly prolonged in the dexamethasone bupivacaine group with a mean duration of 1098.00 ± 195.90 min compared to 464.29 ± 113.75 min in the bupivacaine alone group, p-value < 0.001. Moreover, the dexamethasone bupivacaine group significantly consumed less total tramadol and diclofenac than bupivacaine alone, with a median dose of 0 (0–50) versus 50 (21.25–78.75) mg and 40 (0–50) versus 65 (47.50–77.50) mg, respectively. The median visual analogue scale scores were significantly reduced at 6th, 8th, 12th, and 24th hour in the dexamethasone bupivacaine group. Conclusion: Dexamethasone added to low-dose high-volume of bupivacaine in supraclavicular brachial plexus block significantly prolonged the duration of analgesia.