<p>Immunoglobulin G4-related disease (IgG4-RD) is a systemic immune-mediated condition that can affect multiple organs. Prostatic involvement, although uncommon, may closely mimic prostate cancer due to the presence of a prostatic mass and elevated prostate-specific antigen (PSA) levels. In rare cases, IgG4-related prostatitis occurs in isolation, without evidence of other organ involvement. We present the case of a 78-year-old man initially suspected of having prostate cancer based on a markedly elevated PSA level and prostate magnetic resonance imaging findings. However, prostate biopsy revealed no malignancy, instead showing dense infiltration of IgG4-positive plasma cells. Along with elevated serum IgG4 levels, these findings led to a diagnosis of isolated IgG4-related prostatitis. The patient initially declined treatment, but over the following two years, his prostate volume increased, and his dysuria, characterized by urinary frequency, difficulty urinating, and a sensation of post-void residual urine, worsened. Initiation of prednisolone therapy resulted in normalization of serum IgG4 levels and marked improvement in both prostate swelling and urinary function. A literature review identified only three additional reported cases of isolated IgG4-related prostatitis. This case highlights the potential for prostate-limited IgG4-RD and underscores the importance of including IgG4-related prostatitis in the differential diagnosis of suspected prostate cancer, particularly when biopsy findings are atypical.</p>