Glomerular IgM deposition is commonly shown in idiopathic membranous nephropathy, but the clinicopathological features and outcomes of IMN with IgM deposition are unclear. This single-center prospective cohort study enrolled 210 patients with biopsy-proven IMN from January 2016 to December 2018. Clinicopathological features, treatment responses, and kidney outcomes were compared between patients with and without IgM deposition. In total, 76 (36.2%) patients show glomerular IgM deposition. Patients with IgM deposition were younger (45 +/- 13.30 vs. 50.59 +/- 13.65 years, P = 0.006), had a higher estimated glomerular filtration rate (eGFR) (100.03 [81.31-111.37] vs. 92.67 [74.71-106.63] mL/min/1.73 m(2), P = 0.041), and had a lower proportion of nephrotic syndrome (60.5% vs. 75.4%, P = 0.024) at the time of kidney biopsy. Patients with IgM deposition had a significantly higher proportion of focal segmental glomerular sclerosis (FSGS) lesions (27.6% vs. 13.4%, P = 0.011) and C1q deposition (72.4% vs. 57.5%, P = 0.032). Although the treatments and initial treatment responses were comparable, patients with glomerular IgM deposition had a significantly greater proportion of eGFR decline of >= 5 mL/min/1.73 m(2) per year (log-rank test, P < 0.001) and eGFR decrease of >= 10% from baseline (log-rank test, P = 0.003). Cox regression analysis showed that IgM deposition was an independent risk factor of eGFR decline of >= 5 mL/min/1.73 m(2) per year (HR, 2.442; 95% CI, 1.550-3.848, P < 0.001) and eGFR decline by >= 10% from baseline (HR, 2.629; 95% CI, 1.578-4.385, P < 0.001) during follow-up. IgM deposition in the glomeruli is an independent risk factor for decreased renal function in patients with IMN.