<p>A 63-year-old woman underwent living-donor kidney transplantation three years earlier for end-stage renal disease due to diabetic nephropathy, with her younger sister as the donor. She was prescribed calcium polystyrene sulfonate for the management of hyperkalemia, which had been discontinued two years earlier. At this time, she developed recurrent abdominal and urinary symptoms, which were managed empirically with antibiotics. Four months prior to admission, she noted mucoid vaginal discharge. Pelvic MRI revealed a pelvic mass. She underwent an open biopsy and pathological examination revealed an appendiceal abscess with deposition of calcium polystyrene sulfonate crystals and an associated inflammatory pseudotumor. The lesion resolved with antibiotic therapy and drainage. Calcium polystyrene sulfonate is known to cause gastrointestinal complications; however, it may be associated with the development of inflammatory pseudotumors in rare cases. Surgical resection is generally considered the first-line treatment for inflammatory pseudotumors; however, treatment strategies for unresectable cases have not been established. Herein, we report a rare case of an inflammatory pseudotumor with calcium polystyrene sulfonate crystal deposition that was successfully managed with antibiotics.</p>