Background: Postoperative pain is a common expected problem. Appropriate medications are important to control pain, especially in breast cancer surgery with a high rate of postoperative acute and chronic pain. Objectives: This study aimed to evaluate the effect of intraoperative infusion of dexmedetomidine on acute and chronic pain after simple mastectomy. Methods: This is a double-blind clinical trial study which included 110 eligible patients for simple mastectomy from Shahid Faghihi Educational and Medical Center in Shiraz between October 2018 and May 2019. The patients were randomized into two groups of intervention who were received intraoperative dexmedetomidine 0.4 μg/kg/h, and control group who were received the same amount of placebo (normal saline 0.9%). The severity of acute and incidence of chronic pain were assessed with VAS of pain, and brief pain inventory (BPI), and the number of analgesics (Apotel, Morphine) that patients received in first 24 hours were recorded postoperatively. Collected data were analyzed, and P-values of less than 0.05% were considered statistically significant. Results: Here, 110 patients (55 people in each group) were met the inclusion criteria of the study. Based on the result of this study, the mean VAS score of pain in the intervention group (0.96 ± 0.14) was significantly lower than the control group (4.44 ± 2.25) (P < 0.001). Although the incidence of chronic pain was different between the two groups after three months, this difference was not statistically significant (P = 0.871). Moreover, the number of requests and the amount of analgesia (Apotel and Morphine) in the first 24 hours after surgery in patients in the intervention group was significantly less than patients in the control group (P < 0.001). Also, the time of the first request for analgesia (Apotel, Morphine) in the patients of the intervention group was significantly longer than the control group (P-value = 0.036). Conclusions: According to the finding of this study, the use of dexmedetomidine during simple mastectomy reduces pain, decreases, and delays in requirement of analgesia at the first 24 hours post-operation. However, it did not have a significant effect on chronic pain.