Background Functional endoscopic sinus surgery (FESS) has been proposed as a selected treatment used in patients with chronic sinusitis that have not responded to medical therapy. Due to the nature of the location of endoscopic sinus surgery, even a small amount of bleeding can reduce the operative visibility. The aim of the work was to compare between dexmedetomidine and magnesium sulphate regarding their efficacy as a hypotensive agent in FESS in adult patients to obtain a bloodless surgical field. Patients and methods Sixty patients were randomly assigned into two groups, (D group) for dexmedetomidine (n = 30) and (M group) for magnesium sulphate (n = 30). In (D group), patients received 1 μg/kg dexmedetomidine in 100 ml saline solution as the loading dose 10 min before induction and 0.5–1 μg/kg/h infusion via syringe pump during surgery. In (M group), patients received 40 mg/kg magnesium sulphate in 100 ml saline solution over 10 min as the intravenous loading dose 10 min before induction, with a subsequent 10–15 mg/kg/h infusion. If there is an increase in the arterial blood pressure greater than the targeted MAP (55–65 mmHg), nitroglycerine infusion was started by 0.5 μg/kg/min. The surgeon estimated the quality of the surgical field and recorded it. The total blood loss was measured. In recovery, time to reach Aldrete score ≥ 9 was recorded to fulfill the discharge criteria. Pain score was assessed by the NRS numerical rating score. The time needed to first analgesia requirement was recorded. Sedation score was recorded using Ramsay sedation score. Results There was no statistically significant difference between both groups regarding MAP except at 30 min postoperatively as there was a statistically significant decrease in the MAP among the dexmedetomidine (D group) (p = 0.039). Nitroglycerine was required in eight cases in the magnesium sulphate (M group) to reach the targeted MAP which was statistically significant compared to the (D group). Patients in the (D group) had a statistically significant decrease in heart rate than in the (M group) during and after the operation (p = 0.35). The quality of the surgical field was significantly better among the (D group) (p < 0.05), and accordingly, the surgeon satisfaction was significantly higher in the (D group) (p = 0.001). Time to reach Aldrete score ≥ 9 and time for first analgesic requirement postoperatively were significantly longer in the (D group) (p = 0.023, p = 0.001 respectively). Regarding the Ramsay sedation score (RSS), it was higher in the (D group) which was statistically significant (p ≤ 0.001). Conclusion Dexmedetomidine was more effective than magnesium sulphate to achieve controlled hypotension in patients undergoing FESS. Compared with magnesium, dexmedetomidine offers the advantage of better clarity of the field, surgical satisfaction, less bleeding, and prolonged postoperative analgesia.