A 44-year-old male underwent a deceased-donor kidney transplant 3 years ago and has been taking prednisolone, tacrolimus, and mycophenolate mofetil as immunosuppressive agents. Focal segmental glomerulosclerosis was diagnosed by kidney biopsy 20 years ago. Owing to progressive kidney dysfunction, the patient started peritoneal dialysis 6 years ago, which continued until the kidney transplant. He had a history of partial nephrectomy for the treatment of clear cell carcinoma of the right kidney 11 years ago. The transplanted kidney function had been stable, with a serum creatinine level of 1.42 mg/dL and an estimated glomerular filtration rate of 60 mL/min/1.73 m². Although he exhibited no complaints or symptoms, we conducted an abdominal computed tomography scan owing to his history of malignancy. The scan revealed an unexpected soft-tissue mass around the transplant kidney’s vessels. Magnetic resonance imaging was performed for a detailed characterization of this mass (Fig. 1). In the T2-weighted image, a 2 × 3-cm soft-tissue mass encasing the transplant renal vein was observed, showing hypermetabolism on positron emission tomography-computed tomography. We performed an ultrasound-guided targeted biopsy for histological confirmation and diagnosed a posttransplant lymphoproliferative disorder (CD20+/CD79a+ B lymphocytes forming aggregates within a background of CD3+/CD45R0+ T cells without Epstein-Barr encoding region staining). Positron emission tomography-computed tomography was conducted to evaluate for other lesions or lymph node involvement, confirming the absence of other malignant lesions and local posttransplant lymphoproliferative disorder without lymph node involvement.