A 33-year-old Iranian woman with a history of four Natural Vaginal Deliveries (NVDs) was admitted for evaluation of a 2-month delay in her menstrual cycle in 10th, June 2020. The patient had no special past medical history (including any malignancies in the breast, gastrointestinal tract, or other organs). During the admission, the patient’s vital signs were stable; however, in physical examination, one hard mass in right hypogastric region was found. In the same day, lab tests showed mild normocytic normochromic anemia, while liver and kidney function tests, along with the urine culture test were normal. Tumor markers were checked for the patient; CA19-9 was 104.9 U/L (reference range: 0–33 U/L), and CA125 was 21.3 U/L (reference range: < 35 U/L). In the next day, transvaginal ultrasound revealed a heterogenous mass in posterior cul-de-sac and retrovesical area, which led to the displacement of the urinary bladder and uterus. A mass lesion with dimensions of 154*137*111 mm with mixed density containing cystic, solid, and fat components along with the multiple foci of calcifications were found in right paracolic gutter in Computed Tomography Scan (CT-Scan) in 13th, June 2020. Due to the presentation of a mixed density element on one side and a heterogenous solid element on the other side of the lesion, probability of collision tumor and immature teratoma was proposed. CT-Scan stereotypes showed a right ovarian mass as well as a left ovarian mass which caused a mass effect on uterine. Since the laboratory data suggested a mucinous cyst, an appendectomy was also suggested. On 14th, June 2020, the patient underwent left salpingo-oophorectomy, right ovarian cystectomy, omentectomy, and appendectomy; however, uterus and right ovary were preserved. The histological evaluation of appendix was reported normal. A colonoscopy was performed in to rule out gastrointestinal tract metastasis, which later appeared to be normal. Gross examination of the previously-cut specimen, labeled as left ovarian cyst, on 14th, June 2020 revealed round gray mass with smooth external surface measuring 130*120*120 mm with two lesions with clear margins involving the left ovary. The larger one measuring 120*100*45 mm with solid and cystic cut surface contained thick mucoid material and necrotic tissue. Adjacent cystic space contained sticky yellow material and tufts of hair measuring 70*60*55 mm and showed one elastic projection measuring 30*15*10 mm. The right ovarian cyst was composed of previously-opened multilocular cyst measuring 65*35*35 mm, containing tufts of hair and yellow material without solid component. Focal elastic projection measuring 15*10*5 mm was also observed. The omentum, left fallopian tube, and appendix seemed to be unremarkable. Microscopic slides of smaller cystic lesion showed mature teratoma composed of normal skin tissue with its appendages like hair follicles, sebaceous glands, and subcutaneous adipose tissue. Respiratory mucosa, gastrointestinal mucosa (mostly of colon type), salivary glands, and mature cartilage tissue were also observed. The slides of larger solid cystic mass showed extensive necrosis, back-to-back variable-sized glandular structures, cribriform, and fused glands lined with stratified mucinous columnar epithelial cells, which included pencil-like atypical nuclei, scattered nucleoli, and exhibited prominent mitosis. A 6-month follow-up of the patient by checking tumor markers appeared to be normal, and there were no signs of tumor markers rising.