Introduction: Local anaesthetics used in spinal anaesthesia have a limited duration of action. To prolong postoperative analgesia, adjuvants, mainly opioids, are used. As µ agonist drugs have side effects, other receptor agonists are considered. Nalbuphine is a safe and effective kappa agonist adjuvant. Aim: To compare the analgesic efficacy between fentanyl and nalbuphine adjuvants added to 3 mL of 0.5% intrathecal hyperbaric bupivacaine. Materials and Methods: This prospective, double-blind, comparative study was conducted in 60 patients of either sex belonging to the American Society of Anesthesiologists classes I and II aged 18–65 years undergoing lower limb surgery with entropy monitoring, randomly allocated into two groups. Group F (n = 30) received 0.5% hyperbaric bupivacaine (3 mL) + 25 µg (0.5 mL) fentanyl. Group N (n = 30) received 0.5% hyperbaric bupivacaine (3 ml) + 0.8 mg (0.5 mL) nalbuphine intrathecally. Hemodynamics, entropy, motor and sensory block characteristics, and complications were noted. Results: The nalbuphine group had a significantly longer two-segment regression time of sensory blockade and extended analgesia duration than the fentanyl group. Haemodynamics, entropy, time for onset of sensory and motor blockade and adverse effects were comparable in both groups. Conclusion: Nalbuphine prolongs sensory blockade and postoperative analgesia duration with minimal side effects and is a safe intrathecal adjuvant.