Rationale: Fibroma of tendon sheath is a rare benign tumor that is attached to the tendon sheath. Patient concerns: A 16-year-old boy presented with a painless mass in his right foot, which was initially misdiagnosed as a giant cell tumor of the tendon sheath. Diagnosis: Fibroma of tendon sheath, which exhibited radiographic features similar to those of giant cell tumor of tendon sheath. Excisional biopsy demonstrated spindle-shaped cells and collagen-like stroma. Interventions:The patient underwent excision biopsy. Outcomes: The patient recovered well and showed no signs of recurrence at 6-month follow-up. Lessons: This case provides valuable insights for foot and ankle surgeons. While radiological investigations are helpful in many diseases, histological examination is indispensable for establishment of the final diagnosis. Abbreviations: FTS = fibroma of tendon sheath, MRI = magnetic resonance imaging. GCTTS = giant cell tumor of tendon sheath.