This study aimed to prevent and control the pain after arthroscopy that leads to patient satisfaction, rehabilitation, and return to normal life as soon as possible. It is hypothesized that there is no difference between intra-articular injection of dexmedetomidine and placebo after knee arthroscopy regarding pain level. This double-blind randomized clinical trial was conducted on 70 patients aged 18-60 years who were candidates for elective knee arthroscopic surgery with the American Society of Anesthesiologists Classification I-II. All patients underwent spinal anesthesia equally and were randomly divided into two groups of 35 cases per group. The drug group (D) received 2 μg/kg dexmedetomidine with 0.9% normal saline reached to a volume of 20 ml, and 20 ml of 0.9% normal saline was injected into the knee joint through the cannular sheath in the control group (C). Postoperative pain intensity was recorded 1, 2, 4, 8, 16, 24 h after injection using the Visual Analogue Scale. The time of requesting the first analgesic and the amount of analgesics consumed were recorded after 24 h. There was no significant difference between the two groups in terms of age, height, weight, duration of spinal anesthesia, and duration of surgery (P>0.05). In group D, there was a decrease in postoperative pain, a decrease in the amount of analgesic consumed, and an increase in the time of the first analgesic request, compared to group C (P<0.05). This study showed that intra-articular injection of dexmedetomidine relieved postoperative pain, reduced analgesic consumption, and increased the time of first analgesic request after knee arthroscopy.