Abstract Background: Lupus nephritis (LN) is a major issue that adds a burden on patients with systemic lupus erythematosus (SLE). Immunoglobulin-binding protein 1 (IGBP1) is identified as a phosphoprotein that has been recently reported to be linked to the B-cell receptor complex and regulates differentiation, proliferation, apoptosis, and tolerance of B cells. Its diagnostic and/or prognostic role in LN has been highlighted only recently. Objectives: This study aims to evaluate the relation between serum IGBP1 and SLE disease activity and/or renal activity and to investigate the validity of IGBP1 as a biomarker for LN. Methods: 96 participants were enrolled and divided into three groups: nephritis, nonnephritis, and control groups. The patients with SLE were diagnosed according to the Systemic Lupus International Collaborating Clinics classification (SLICC) criteria. The serum IGBP1 level was assayed using an enzyme-linked immunosorbent assay (ELISA). Assessments were conducted using the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2k) and renal biopsy for LN patients. Results: The nephritis and nonnephritis groups had higher IGBP1 levels than the controls; the nephritis group had the highest serum IGBP1 levels (p < .001). Significant correlations were found between IGBP1 levels and proteinuria (r = 0.568, p = .001) and renal SLEDAI (r = 0.475, p = .006) in the nephritis group; on the other hand, the correlation of serum IGBP1 levels with SLEDAI-2K was non-significant for both groups (nephritis and nonnephritis groups). The IGBP1 levels were significantly different among histopathologic classes (p < .001), with class V showing the highest level. Moreover, it showed a significant positive correlation with the pathologic activity index. Compared with renal SLEDAI for identifying active renal affection in patients with SLE, the serum IGBP1 level with a cut-off value of 547.45 ng/mL is a valid biomarker for detecting active nephritis with 93.8% sensitivity and 96.9% specificity. Conclusion: The serum IGBP1 levels were high in patients with LN and were positively correlated with the pathologic activity index. The serum IGBP1 level of 547.45 ng/mL is a valid biomarker for detecting active nephritis. Thus, we recommend that clinicians monitor the serum IGBP1 level of patients with SLE to detect LN.