The diffuse sclerosing variant of papillary thyroid carcinoma (DSV-PTC) is a rare tumor with aggressive behavior that requires aggressive treatment. Despite characteristic clinical and histological features that easily permit diagnosis, pre-operative fine-needle aspiration cytology (FNAC) diagnosis is often challenging and thus delays diagnosis. We describe the cytological features of a case of DSV-PTC diagnosed by FNAC in a 30-year-old woman presenting with an ill-defined mass in her neck lasting for 2 months. Ultrasonograpy revealed a heterogeneous enlargement of both thyroid lobes suspicious for a lymphoproliferative syndrome. Flow cytometry showed a suspect B-lymphocyte population. FNAC showed in five out of six slides an overwhelming presence of slightly atypical monomorphic small lymphocytes. The remaining slide showed syncytial tissue fragments of follicular cells with nuclear enlargement and pleomorphism, irregular nuclear membrane, grooves with scattered intranuclear inclusions, squamous metaplastic epithelium, and abundant psammoma bodies. A diagnosis of DSV-PTC was rendered and confirmed by total thyroidectomy and lymph node dissection. Our report supports the possibility of obtaining a preoperative diagnosis of DSV-PTC by FNAC. In the case of diffuse thyroid enlargement, adequate sampling of the thyroid and the presence of the combination of features described in our case permitted the diagnosis of this PTC variant.