Ankylosing spondylitis (AS) is a chronic inflammatory disease affecting primarily the axial skeleton. Renal involvement occurs in 5–13% of AS patients, with membranous nephropathy (MN) being particularly rare, with only 10 cases reported in PubMed. The article describes the case of a 37-year-old man with an 11-year history of AS who developed nephrotic syndrome in the stabilized phase of the disease. Renal biopsy confirmed MN with IgG and C3 deposition along the glomerular capillary walls and electron-dense subepithelial deposits. After exclusion of secondary causes, treatment with rituximab was chosen as the initial immunosuppressive therapy. Six months after treatment, 24-hour urinary protein excretion decreased from 12.9 g to 836 mg and serum albumin increased from 21.4 g/L to 35.4 g/L, resulting in partial remission. This case provides clinical evidence of an association between AS and MN and demonstrates the efficacy and safety of rituximab in this rare scenario.