Vaccine-induced hyper-metabolic lymph nodes following COVID-19 mRNA vaccination have been reported clinically. We report a case of a kidney recipient who developed an enlarged abdominal mass after receiving the booster dose of the COVID-19 mRNA vaccine. A 79-year-old male patient who received a booster shot of mRNA COVID-19 vaccine earlier in the day, presented to the emergency department with acute pain and weakness in his lower extremities .He had undergone kidney transplantation for end-stage renal disease of unknown origin 25 years ago. The patient had no history of an adverse reaction to a vaccination before receiving the booster dose of the mRNA COVID-19 vaccine. In the evening after receiving the booster dose, the patient experienced a rapid onset of increasing left inguinal pain, scrotal swelling, and left leg weakness. Computerized tomography showed subcapsular and perirenal fluid collection around the transplanted kidney. Swelling with hydroureter and a high density of the transplanted kidney in the left iliac fossa were observed. Percutaneous nephrostomy was performed on the transplanted kidney. Analysis of the percutaneously drained abdominal fluid revealed the presence of white blood cells, with lymphocytic dominance.It was more commonly observed after receipt of COVID-19 mRNA vaccines, comparing to protein-based vaccines. The mechanism of action for this vaccination-associated reactive lymphadenopathy includes a rapid B-cell proliferation by the mRNA vaccine. Herein we report a patient who presented with ipsilateral lymphadenopathy that was mistaken for a urinoma. This mistake was undoubtedly due to the large size of the lymphadenopathy and the fact that it filled the peritoneal space surrounding the graft kidney, infiltrated the psoas muscle, and caused ureteric compression and hydronephrosis. Lymphadenopathy is an adverse reaction to the Pfizer and Moderna vaccines. Therefore, immunocompromised patients should be educated on the incidence and potential severity of lymphadenopathy after booster vaccination.