Objective <p>Oral lichen planus (OLP) is a chronic inflammatory disease of the oral mucosa with a recognized potential for malignant transformation into squamous cell carcinoma (SCC). This report aims to describe a rare and well-documented case of OLP that underwent malignant transformation after a 27-year follow-up, emphasizing diagnostic, histopathological, and clinical aspects.</p> Case Report <p>A 38-year-old male patient with a previous diagnosis of cutaneous lichen planus presented with reticular and erosive OLP lesions involving the buccal mucosa, gingiva, and tongue. Histopathological analysis confirmed the diagnosis of OLP. The patient initially showed clinical improvement after topical corticosteroid therapy and was advised to attend six-month follow-up visits. However, adherence was irregular, with the patient returning only during symptomatic exacerbations. After 25 years, a biopsy of the left buccal mucosa revealed oral lichen planus associated with severe epithelial dysplasia. Two years later, the lesion progressed to a firm infiltrative nodule, and histopathological examination confirmed well-differentiated SCC. The patient underwent oncologic resection of the primary tumor with therapeutic cervical lymph node dissection, followed by adjuvant radiotherapy, due to the presence of metastatic lymph nodes. Despite treatment, local recurrence occurred six months later, and the disease progressed to death.</p> Conclusion <p>This exceptionally long-term case illustrates the malignant potential of OLP, even in patients without traditional risk factors such as tobacco or alcohol use. It highlights the importance of continuous and structured monitoring of OLP, especially in cases presenting erosive or atrophic lesions, as well as the need for patient compliance and clinician vigilance. Long-term follow-up and the use of predictive biomarkers are essential to enable early detection of malignant transformation and improve patient outcomes.</p>