Background: Hypotension is commonly encountered following spinal anaesthesia, resulting from sympathetic blockade and decreased cardiac output. Perfusion index (PI) is a valuable objective during anaesthetic-practice for early prediction of hypotension. This study aimed to compare variations in upper limb and lower limb PI following subarachnoid block in femur surgeries and also investigate its correlation with intraoperative hypotension and sensory blockade. Methods: In this prospective observational study, 100 American Society of Anaesthesiologists (ASA) I & II, aged 20-50 years, patients scheduled for elective femur surgeries were enrolled. PI and blood pressure were noted at baseline, at every minute for five minutes, and every five minutes till 30 minutes in both upper limb and lower limb, after administration of subarachnoid block tailored according to patient's weight and duration of surgery. Incidence of hypotension was noted at all observation time points. Spearman's rank correlation coefficient was used to assess correlation between baseline PI and the number of episodes of hypotension and onset of sensory blockade. A Receiver Operating Characteristic (ROC) curve was obtained for baseline PI compared with hypotension episodes of 100 patients and a p-value of <0.05 was considered as statistically significant. Results: The incidence of hypotension is higher with baseline preoperative PI >3.2. The intraoperative increase in PI of lower limb is associated with a simultaneous decrease in PI of the upper limb and increased incidence of episodes of hypotension at various time intervals. Also, there was no correlation between sensory block time and perfusion index. The ROC curve revealed that PI discriminated well between patients who developed hypotension versus those who did not; it yielded a new baseline PI value of 3.2 as cut-off point for predicting hypotension in patients undergoing femur surgeries under subarachnoid block with a sensitivity and specificity of 82%and 79% respectively. Conclusion: PI > 3.2 at baseline has a higher risk of hypotension following subarachnoid block in femur surgeries.