Background: Endometrial cancer represents the most prevalent malignant genital tract neoplasm in high-income countries and is the second most common cancer worldwide following cervical cancer. Endometriosis is a benign condition wherein endometrial glands and stroma are found outside the uterine cavity. Case Presentation: During a routine care and ultrasound examination of the uterus and adnexa of a 64-year-old woman, an increased endometrial thickness (22 mm) was noted. In 2023, according to ultrasound report, the patient underwent diagnostic curettage with immunohistochemistry, revealing a pathological diagnosis of endometrial cancer (endometrioid adenocarcinoma) with positive staining for p16, estrogen receptor (ER), and vimentin. Subsequently, after one week, she underwent complete surgical staging. Extensive superficial endometriosis disseminated in the pelvis and vulva was noted during surgery and preoperative examinations. Final pathology confirmed a well-differentiated typical endometrioid carcinoma (grade 1) with 40% myometrial invasion and positive lymphovascular invasion. The patient was considered to be at stage 1A. Conclusion: Despite some studies suggesting an unclear association between endometriosis and endometrioid or clear-cell ovarian cancers, the correlation between endometriosis and endometrial cancer and its prognosis remains ambiguous. Additionally, although infertility has been linked to both endometrial cancer and endometriosis in various studies, the presented case exhibited no signs of infertility. Extensive pelvic endometriosis with vulvar involvement was present, yet the patient did not exhibit any symptoms. This is in contrast to the typical initial manifestation of endometrial cancer, which is abnormal uterine bleeding. The patient's condition was incidentally detected through routine care due to an abnormal increase in endometrial thickness, prompting this presentation.