Uterine fibroids are the most common benign tumors in women. The increasing incidence of uterine fibroids has led to their increasingly common occurrence in young patients who have not yet achieved reproductive function. This has also led to an increase in the number of cases of uterine leiomyosarcoma diagnosed after organ-preserving surgery for uterine fibroids. Improving the quality of diagnostic examinations for patients with uterine fibroids and, accordingly, improving preoperative diagnostic methods for uterine sarcoma have long been a pressing issue in gynecology. Unfortunately, it remains virtually impossible to distinguish between uterine fibroids and leiomyosarcoma preoperatively. Despite the rapid advancement of imaging techniques (magnetic resonance imaging, computed tomography, and positron emission tomography with computed tomography), suspecting a malignant tumor of the uterine body is rare. This article presents a clinical case of uterine leiomyosarcoma diagnosed in a 25-year-old patient who had not realized her reproductive function. Uterine fibroids were first diagnosed at age 21; despite radical surgical treatment, a relapse was diagnosed shortly thereafter. During a repeat surgical procedure, the uterine tumor was classified as low-grade leiomyosarcoma, necessitating radical treatment—hysterectomy—despite the patient’s young age. Leiomyosarcoma is characterized by an aggressive course and poor prognosis, as clearly demonstrated in the presented clinical case. Despite the fact that the patient underwent laparotomy with ablastic retrieval of macroscopic specimens, multiple tumor disseminations were detected within a short period of time (6 months) in the abdominal cavity, lungs, bones, and anterior abdominal wall. Combined drug therapy and surgical treatment of recurrent tumors allowed for successful treatment of the patient with multiple metastases from uterine leiomyosarcoma for 7 years. Despite the rare nature of uterine leiomyosarcoma, patients with uterine fibroids, even young patients, require oncological vigilance and close medical attention at all stages—from outpatient examination up to the choice of treatment strategy, the extent and approach of surgery, and postoperative follow-up.