Background To evaluate the failure pattern and identify predictors of locoregional control in lateralized buccogingival cancer after postoperative radiotherapy (RT) at a single institution. Methods We retrospectively reviewed the clinical data of 150 patients with lateralized oral squamous cell carcinoma, including carcinoma of the buccal mucosa, gingiva and retromolar trigone. All patients underwent radical surgery followed by postoperative RT with or without concurrent chemotherapy. We registered planning computer tomography images with images obtained at recurrence and categorized the failure pattern as in-field, marginal, or out-field recurrence. Results The median follow-up duration was 47 months (range, 2–131 months). Twenty-eight patients (19%) experienced locoregional failure, including 20 local failure, 5 regional failure and 3 with both. Among the 24 patients who had image studies at recurrence, 15 patients had in-field recurrence, 5 were marginal recurrence and 4 were out-field recurrence. Seven patients (5%) had contralateral neck failure. Four of 5 patients with marginal failure had recurrent tumors in the infratemporal fossa. In multivariate analysis, extracapsular spread and positive or close surgical margin were associated with poor locoregional control. Conclusion Local in-field recurrence is the most common failure pattern in lateralized buccogingival cancer after postoperative RT. The infratemporal fossa is a risk area for marginal failure and should be encompassed adequately in the postoperative RT field. Extracapsular spread and positive or close margin are predictors of locoregional control for lateralized oral cancer. Patients exhibiting such adverse features require more aggressive treatment.