We report a 67-year-old man who presented with poor dietary intake and fatigue. Laboratory tests showed leukopenia, antinuclear antibody (ANA) positivity, anti-dsDNA antibody (A-dsDNA) and anti-Smith antibody (anti-Sm) negativity, decreased C3 and C4, elevated serum immunoglobulin G (IgG), IgG4, and creatinine, and 1.25 g urinary protein at 24 hours. As his condition worsened, re-examination showed thrombocytopenia and A-dsDNA positivity, and renal biopsy pathology showed IgG4-related tubulointerstitial nephritis. The final diagnosis was IgG4-related disease (IgG4-RD) with systemic lupus erythematosus (SLE). His condition improved with glucocorticoid (GC) combined with hydroxychloroquine (HCQ) and mycophenolate mofetil (MMF) treatment. This case highlights that IgG4-RD and SLE may occur successively or co-exist and may convert into each other.