Background: Shivering is a common and distressing complication during cesarean sections performed under spinal anesthesia, posing risks to both maternal and fetal well-being. Effective shivering prevention is essential to enhance perioperative outcomes. Intrathecal fentanyl and intravenous ondansetron are commonly used pharmacological agents for this purpose, yet their comparative effectiveness has not been conclusively determined. Objective: This study aimed to compare the efficacy of intrathecal fentanyl versus intravenous ondansetron in preventing shivering in full-term pregnant women undergoing cesarean section under spinal anesthesia. Method: A randomized controlled comparative study was conducted at SV Medical College, Tirupathi, Andhra Pradesh, between February 2023 and August 2023. A total of 250 full-term pregnant women scheduled for cesarean section were randomly assigned into three groups. Group F (n=83) received intrathecal fentanyl (15 μg) with spinal anesthesia, Group O (n=83) received intravenous ondansetron (8 mg) added to the usual saline solution, and Group C (n=84) served as the control, receiving only IV fluid and intrathecal bupivacaine without fentanyl. The incidence of shivering during the procedure was observed and recorded for all participants. Results: Shivering occurred in 15 of the 83 women in Group F, resulting in an incidence rate of 18.1%. In Group O, shivering was observed in 23 of the 83 women, corresponding to an incidence rate of 27.7%. The control group (Group C) exhibited the highest incidence, with shivering occurring in 38 of the 84 women, representing 45.2% of the group. The reduction in shivering incidence was statistically significant in Group F compared to both Group O (p<0.05) and Group C (p<0.01). Conclusions: Intrathecal fentanyl significantly reduces the incidence of shivering in cesarean sections under spinal anesthesia compared to intravenous ondansetron and no shivering prevention. With an incidence reduction of 59.9