Renal involvement in ankylosing spondylitis (AS) is uncommon and usually secondary to amyloidosis or analgesic nephropathy. A 41-year-old female patient who was diagnosed with human leukocyte antigen-B27-positive AS 16 years ago was admitted to the rheumatology outpatient clinic with pretibial and pedal edema. She had nephrotic-range proteinuria. Renal biopsy findings confirmed the diagnosis of membranous nephropathy. Extensive malignancy evaluation was negative. Edema regressed, renal functions stabilized, and proteinuria reduced within two months after immunosuppressive therapy.