A collision tumor involving metastasis to a pituitary adenoma is rare. We describe a case of a 68-year-old Bidayuh woman with underlying treatment-responsive lung adenocarcinoma, who presented with mass effect, panhypopituitarism and polyuria. Her initial imaging study reported pituitary macroadenoma, and she was treated with hormone replacement therapy. She then underwent transsphenoidal tumor debulking surgery with subsequent histopathological findings of a collision tumor of an adenocarcinoma with metastasis to a non-functioning pituitary adenoma.