Introduction: Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage. Spinal anesthesia using local anesthetics like hyperbaric bupivacaine is one of the most popular techniques for both elective and emergency surgical procedures. One disadvantage with spinal anesthesia is relatively short duration of action. Postoperative pain relief provides comfort to the anxious patients and improve their morale and mobility there by contributing to a rapid and complete recovery. Aims: Study done on IV. Magnesium Sulfate, in patients given spinal anesthesia, for postoperative analgesia. Materials and Methods: This prospective study was conducted on 50 adult patients of ASA physical status 1 & 2 in the 18–60 age group, of either sex, posted for elective lower limb orthopedic surgeries under spinal anesthesia over a period of 12 months. Patients were randomly divided on an alternative basis into groups of 25 each. GroupM-Patients receiving IV MgSO4/infusion, Group -S-Patients receiving isotonic saline. Results: The mean height and mean weight in either group were identical. The type of surgeries performed were almost identical. There was no significant difference in hemodynamic variables (mean arterial pressure and heart rate) during the intra-or-postoperative period. Perioperative mean arterial pressure. Time of first pain medication in Group M was 334 min whereas in Group S was 227 min. This was statistically significant (p < 0.001). VAS was statistically significant at the end of 3 and 6 hours (p < 0.001), but it was insignificant at the end of 12 hours (p > 0.05). The cumulative requirement in 24 hrs of both tramadol and diclofenac was statistically significantly less for group M vs. S (p = <0.05). Postoperative serum Mg concentrations in Group M were significantly higher than those in Group S (p < 0.001 immediately after surgery, and at 1 and 24h after surgery. However, all patients in Group M had a serum Mg concentration in the normal range 24h after surgery. Conclusion: IV Magnesium sulfate infusion in spinal anesthesia decreases intra-operative hemodynamic variabilities, prolongs duration of analgesia and improves the quality of analgesia in the early postoperative period with better hemodynamic stability. It also decreases the postoperative analgesic requirements, thus it can be used as a beneficial additive for prolonging spinal anesthesia.