We found a case of a papillary thyroid carcinoma that was accompanied by a familial adenomatous polyposis (FAP) in a 29-year-old female. She had hundreds of adenomas inside the entire colon and a congenital hypertrophy of the retinal pigmented epithelium (CHRPE). The patient underwent a total thyroidectomy and a central compartment neck node dissection. Gross examination of the thyroid identified two solid and cystic lesions. The pathological finding of thyroid cancer revealed a mixture of a peculiar nuclear clearing, cribriform, morula formation, trabecular and papillary pattern. The patient's brother had undergone a total colectomy due to FAP at the age of 25. Genetic analyses of the patient's family members revealed that she and her brother had the same germline mutation, in which five nucleotides (AAAGA) were deleted from codon 1309 of the adenomatous polyposis coli (APC) gene exon 15. Strong and frequent immunoreactivities of β-catenin and p53 were evident in the tumor tissue. At the time of writing, a preventive colectomy was still under consideration for the patient. Genetic counseling was given to the other family members, who were not attacked by this disease, in order to allay their fears of cancer.