Background and Objectives: The aim of this study was to investigate the efficacy of a single preoperative dose of deflazacort on pain, swelling, and trismus after impacted lower third molar surgery. Materials and Methods: This randomised, prospective, double-blind, split-mouth clinical study included 26 healthy individuals with bilaterally impacted lower third molars. Group 1 was given a placebo (single-dose vitamin C tablet), and group 2 was given a single 30 mg dose of deflazacort 1 h prior to surgery. Pain was evaluated using the visual analogue scale for 1 week postoperatively. Oedema (in mm) and trismus (in mm) were evaluated preoperatively and on postoperative days 2 and 7. The Mann–Whitney U test was applied for group analyses. p values < 0.05 were considered statistically significant. Results: Postoperative pain scores were significantly lower in the deflazacort group at the 6th and 12th hours after surgery (p < 0.05). There were no significant differences in trismus between the groups at any time point (p > 0.05). There was less oedema in the deflazacort group on postoperative days 2 and 7, without any statistically significant difference (p > 0.05). Conclusions: A single preoperative dose of 30 mg deflazacort was found to be clinically effective in reducing pain and oedema after extraction of impacted lower third molars.