Objectives: Dexamethasone is a steroid that has been widely used during third molar surgery to reduce acute complication associated with mandibular third molar surgery. This study compared the effect of preoperative versus postoperative oral dexamethasone on swelling, pain and trismus following impacted mandibular third molar surgery. Materials and Methods: A total of 138 subjects with impacted mandibular third molar referred for surgical extraction at the Oral and Maxillofacial Surgery Department of the Lagos State University Teaching Hospital Ikeja were enrolled for the study and randomised into two groups. Group A subjects had 8mg of oral dexamethasone tablets one hour before surgical extraction while group B subjects had 8mg of oral dexamethasone tablets immediately after surgery. Pain, swelling and trismus were evaluated in both groups at days 2, 5 and 7. Statistical analysis was carried out using SPSS (IBM version 23). P-value was set at <0.05. Results: There were 61 males and 76 females with an age range of 21-57 years and an average age of 30.7 years and 29.6 years for groups A and B respectively. The results of the study indicated that there was no statistical difference in the reduction of pain, swelling, and trismus after mandibular third molar surgeries in both groups. Conclusion: Therefore, dexamethasone can be used either pre-operatively or postoperatively in minimising postoperative swelling, pain and trismus following mandibular third molar surgery provided there are no contraindications to its use.