Pigmented villonodular synovitis (PVNS) following total knee arthroplasty (TKA) is a rare condition. We reported a case of PVNS following TKA; initially, arthroscopic synovectomy was performed; however, recurrence of PVNS happened. We performed second arthroscopic synovectomy and moderate dose radiotherapy (total dose 44 cGy/22 fractions to posterior fossa, 36 cGy/18 fractions to knee) for the patient. Unfortunately, distal femur osteonecrosis with femoral and tibial component loosening happened 2 months after completing the radiotherapy procedure. Revision TKA with Legacy Constrained Condylar Knee was done. Radiotherapy is not recommended for recurrent PVNS after TKA.