Aim: It was to evaluate the efficacy of dexamethasone (Dx) of a single dose of 8 mg dexamethasone injected pre-operatively into the pterygomandibular space in reducing post-operative pain, swelling, and limited mouth opening following lower third molar surgery. Subjects and Methods: A prospective, randomized, split mouth study involving 62 surgical extractions of lower third molars in 31 patients. range (20 – 35 years) with similar bilaterally impacted lower third molars. Sites from the study group (SG) received single dose of 2ml of 4mg Dx preoperatively, while those in the control group (CG) received a placebo (the same volume of sterile saline solution). Previous history of significant medical condition, drug allergy, or infection that may contraindicates the use of Dx was excluded. Results: Significant reduction in swelling, pain and total postoperative analgesic consumption was observed in SG than CG on the 2nd day. Also, there were differences between both groups regarding to the interincisal distance (IID) measurements on the 7th day. Conclusion: Both groups showed acceptable results, however, Dx injection seems to be the appropriate treatment for rapid recovery of range of IID, reduction of swelling, and relief of pain following impacted lower 3rd molar extraction.