Background: Struma ovarii (SO) is a rare phenomenon. In addition, it can be identified from the histological assessment and may show the characteristics of Hashimoto’s thyroiditis in rare cases. In several cases, malignant stroma ovarii is found to produce hyperthyroidism while very few cases are reported with Hashimoto’s thyroiditis. The present study reported SO co-incident with Hashimoto’s thyroiditis. It should be mentioned that accurate monitoring of any signs of hypothyroidism is highly important after the surgery. Case Report: A 21-year-old patient with oligomenorrhea, hypogastric pain for the last year, dyspareunia and dysmenorrhea with a right adnexal cystic focus, and a bright echogenic focus referred to Al-Zahra hospital, Rasht. Histopathologic examination was compatible with the diagnosis of mature cystic teratoma containing SO co-incident with Hashimoto’s thyroiditis while the patient was in a good general appearance 2 days after the surgery. Regarding the subclinical hypothyroidism and due to her willingness to conceive, the clinician administered levothyroxine and thus the patient was in a stable state within 6 months. Conclusion: In general, it is recommended to consider SO in women with ovarian mass and impaired thyroid function.