Objective: This case report highlights the diagnostic challenges of non-traumatic myositis ossificans (MO), a rare benign heterotopic ossifying lesion, and emphasizes its potential to mimic malignancy. Case Presentation: An 18-year-old female with no trauma history presented with a one-month history of progressive right calf pain, ankle weakness (4/5 strength), and difficulty ambulating. Initial ultrasound suggested a hematoma, but the persistence of symptoms prompted advanced imaging. CTA revealed arterial extravasation, while MRI identified a heterogeneously enhancing soleus muscle mass, raising suspicion for sarcoma. Core biopsy demonstrated focal bone formation, mitotically active stroma, and a "zone phenomenon" (central cellularity with peripheral osteoid maturation), pathognomonic for MO. Results: Histopathology confirmed non-traumatic MO, avoiding unnecessary interventions. The absence of trauma, atypical soleus involvement, and imaging overlap with sarcoma underscored diagnostic complexity. Multidisciplinary collaboration (vascular surgery, radiology, orthopedics) and biopsy were critical to exclude malignancy. Conclusion: Non-traumatic MO, though rare, should be considered in soft-tissue masses regardless of trauma history. Early histologic evaluation is essential to differentiate it from sarcoma and prevent iatrogenic harm. This case underscores the importance of recognizing MO’s variable presentation and the role of multidisciplinary teamwork in guiding conservative management. Further research is needed to elucidate the incidence and pathophysiology of idiopathic MO.