Introduction Osteoma of the mastoid region is an uncommon benign tumor of bone. Osteomas may occur in the external auditory canals but are reported to be very rare in the mastoid region, with about 150 cases reported in literature. Case Report A 20-year-old man presented with painless lump behind the right ear since last 2 years. On examination a 3x3 cm tumour was present in the right post-auricular region. HRCT scan showing hyper-attenuating area close to the right mastoid cortex, measuring approximately 3x3cm, spherical, with cleavage areas between the tumour and the mastoid external cortex. Surgical excision was performed on cosmetic grounds. Postoperative period was uneventful. Discussion Osteoma over the mastoid region is rare and mainly asymptomatic with excellent prognosis after complete removal of the lesion.