Cowden syndrome is a rare autosomal dominant disorder that is characterized by multiple hamartomas in a variety of tissues and this is associated with germline mutations in the phosphatase and tensin homologue (PTEN) gene, which is the tumor suppressor gene located on chromosome 10q23.3. It is characterized by multiple hamartomatous neoplasms of the skin, oral mucosa, gastrointestinal (GI) tract, bones, central nervous system, eyes, and genitourinary tract. Cowden syndrome does not have increased risk of GI malignancy; however, it has an increased risk of breast, thyroid and endometrial cancer development. Herethe authors report a rare case of Cowden syndrome incidentally diagnosed from multiple gastric polyposis. A 29-year-old woman presented with multiple gastric polyps. The laboratory results were normal except for mild anemia, with a hemoglobin level of 11.9 g/dL. Esophagogastroduodenoscopy revealed multiple gastric, duodenal polyps and esophageal acanthosis. Colonoscopy revealed possible hamartomatous polyps in the rectum. Under the suspicion of Cowden syndrome, sonography of the thyroid and breasts was carried out, which revealed multiple thyroid masses. Subsequent fine-needle aspiration biopsy revealed the presence of clusters of follicular epithelial cells, and due to the possibility of malignancy, the patient underwent total thyroidectomy. The pathology was reported as invasive follicular carcinoma. A gene study by direct sequencing showed the presence of a PTEN mutation (c.633C > A /p.Cys211*). Cowden syndrome is a rare autosomal dominant disorder that is characterized by multiple hamartomas in a variety of tissues and this is associated with germline mutations in the phosphatase and tensin homologue (PTEN) gene, which is the tumor suppressor gene located on chromosome 10q23.3. It is characterized by multiple hamartomatous neoplasms of the skin, oral mucosa, gastrointestinal (GI) tract, bones, central nervous system, eyes, and genitourinary tract. Cowden syndrome does not have increased risk of GI malignancy; however, it has an increased risk of breast, thyroid and endometrial cancer development. Herethe authors report a rare case of Cowden syndrome incidentally diagnosed from multiple gastric polyposis. A 29-year-old woman presented with multiple gastric polyps. The laboratory results were normal except for mild anemia, with a hemoglobin level of 11.9 g / dL. Esophagogastroduodenoscopy revealed multiple gastric, duodenal polyps and esophageal acanthosis. Colonoscopy revealed p Under the suspicion of Cowden syndrome, sonography of the thyroid and breasts was carried out, which revealed multiple thyroid masses. Subsequent fine-needle aspiration biopsy revealed the presence of clusters of follicular epithelial cells, and due to the The pathology was reported as invasive follicular carcinoma. A gene study by direct sequencing showed the presence of a PTEN mutation (c.633C> A / p. Cys211 *).