Supraclavicular brachial plexus block is a common regional anesthesia technique used for upper extremity orthopedic surgeries. The addition of adjuvants to local anesthetics can prolong the duration of analgesia and improve the quality of the block. This prospective, randomized, double-blind study aimed to investigate the impact of dexamethasone on the efficacy of a lignocaine and ropivacaine mixture for ultrasound guided supraclavicular block. A total of 80 patients scheduled for major forearm orthopedic surgery were randomly assigned to receive either a mixture of lignocaine 2% and ropivacaine 0.5% (control group) Group A or the same local anesthetic mixture with the addition of dexamethasone 8 mg (dexamethasone group) Group B. The primary outcome was the duration of analgesia, defined as the time from block placement to the first request for rescue analgesia. Secondary outcomes included the onset time of sensory and motor block, the quality of the block, and the incidence of adverse events. The results showed that the addition of dexamethasone significantly prolonged the duration of analgesia compared to the local anesthetic mixture alone (135 ± 10 min vs. 170 ± 15 min, respectively, p < 0.001). Addition of dexamethasone to mixture of lidocaine and ropivacaine in supraclavicular block results in prolonged duration of sensory and motor block and also improves quality of analgesia without affecting its onset.