We report the case of a 45-year-old man who presented with a 4-year history of midline neck swelling associated with recent onset respiratory distress. Local examination showed a lobulated diffuse thyroid enlargement. A subtotal thyroidectomy was performed. The right lobe weighed 225 g and the left lobe weighed 130 g. Multiple sections from both the lobes revealed diffuse infiltration of the stroma by mature adipose tissue. There was no evidence of amyloid deposits or papillary carcinoma. A final diagnosis of diffuse lipomatosis of the thyroid gland was rendered. Lipomatosis or adenolipomatosis of the thyroid gland is an extremely rare entity. The various differential diagnoses of fat in the thyroid include benign entities like amyloid goiter, adenolipoma, lymphocytic thyroiditis, intrathyroid thymic or parathyroid lipoma and malignant tumors like liposarcoma and encapsulated papillary carcinoma. Adequate clinical details and a thorough histopathological examination are mandatory for diagnosis.