Background: Spinal anaesthesia is widely used for lower abdominal and lower limb surgeries but is frequently complicated by hypotension and bradycardia, leading to significant perioperative morbidity. The Bezold–Jarisch reflex, mediated via serotonin (5-HT₃) receptors, is implicated in this response. Ondansetron, a 5-HT₃ antagonist routinely used for prophylaxis of postoperative nausea and vomiting. Aim: To evaluate the efficacy of prophylactic intravenous ondansetron in reducing the incidence of spinal anaesthesia-induced hypotension compared with placebo. Materials and Methods: This prospective, randomised, double-blind, comparative study was conducted on 76 ASA I–II patients aged 18–60 years undergoing elective non-obstetric surgeries under spinal anaesthesia. Patients were randomised into two groups: Group A (Control, n=38) received 10 ml of normal saline, while Group B (Ondansetron, n=38) received 4 mg ondansetron diluted in 10 ml saline, administered five minutes before spinal anaesthesia. All patients were preloaded with Ringer’s Lactate (20 ml/kg). Spinal anaesthesia was performed with 3.0 ml of 0.5% hyperbaric bupivacaine at L3–L4. Hemodynamic parameters (SBP, DBP, MAP, HR, SpO₂) were recorded at baseline and at regular intervals up to 30 minutes. Results: The incidence of hypotension was significantly lower in the ondansetron group (28.9%) compared with controls (55.3%, p=0.02). Bradycardia (7.9% vs. 26.3%, p=0.03), nausea (7.9% vs. 34.2%, p=0.005), and mephentermine requirement (21.1% vs. 47.4%, p=0.01) were also reduced. SBP, DBP, and MAP were consistently better maintained with ondansetron. No significant differences were observed in vomiting or shivering. Conclusion: Prophylactic intravenous ondansetron (4 mg) significantly reduces the incidence of spinal anaesthesia-induced hypotension, bradycardia, and nausea while improving hemodynamic stability and reducing vasopressor use. Ondansetron may serve as a safe and effective adjunct in routine anaesthetic practice.