Pituitary adenoma with ossification is a rare histological variant. Previously there have been four cases reported in the literature. Here a case of pituitary prolactin-producing adenoma with bone formation in a 21-year-old woman is described. The patient had irregular menstruation for three years. MRI revealed an unusual 1.5 cm3 ovoid nodule with partial shell-like structure showing heterogeneous signals. The pre-operative prolactin serum level was 258.78 ng/mL. The patient was operated through the trans-sphenoidal pathway under general anesthesia. Histologically, the tumor was parenchymal and mostly replaced by the well-differentiated lamellar bony tissue. Sheets of tumor cells interweaved with the mature lamellar bone trabeculae showing no cellular atypia. The cytoplasm of the adenoma cells was slightly eosinophilic and the myelo-adipose metaplastic foci were also found within the parenchyma. Immunohistochemical staining of tumor cells showed positive expressions of prolactin, synaptophysin and chromogranin A in the cytoplasm of the tumor cells. Meanwhile, negative expressions of S-100, epithelial membrane antigen, GFAP and other pituitary hormones were also demonstrated. As a rare histological variant of pituitary adenoma, the current case of pituitary prolactin producing adenoma with ossification is reported. It is speculated that the ossification may be derived from the osteo-metaplasia of mesenchymal fibroblasts resulting from the effects of both secondary ischemia by the outgrowth of the tumor and/or the autocrine effect of prolactin in this case. The bony shell structure may limit the growth of pituitary adenoma.