Ectopic soft tissue calcification (ESTC), a rare clinical condition, causes tissue and organ damage. It is associated with chronic renal failure, hyperparathyroidism, and malignant neoplasms, including multiple myeloma, and it is reportedly resistant to treatment. Here, we present the case of a 71-year-old male with multiple myeloma who had rapid ESTC in the lung. He had developed hypoparathyroidism secondary to thyroidectomy. During the course of our observation, he rapidly developed ectopic pulmonary calcification approximately 2 weeks after acquiring an infection. There was no evidence of further progression of multiple myeloma after the onset of ESTC, and treatment with ferric citrate hydrate and precipitated calcium resulted in immediate improvement of his pulmonary signs. We recommend cautious monitoring for patients with multiple myeloma and hypoparathyroidism to detect the onset of ectopic calcification. In addition, low blood phosphorus levels should be effectively treated.