A 65-year-old Japanese man was admitted to our hospital with fever and inflammation of the right ankle. We initiated antibiotics on suspicion of cellulitis. After no clinical improvement, we performed magnetic resonance imaging, which showed a fluid collection in the flexor hallucis longus (FHL) tendon sheath. Synovial fluid analysis revealed monosodium uric crystals. Final diagnosis was FHL tendonitis secondary to gout proven by synovial fluid analysis. To our knowledge, this is the first case report of FHL tendonitis caused by gout. When ankle inflammation is examined in clinical situations, FHL tendonitis caused by gout should be considered.