Periostin is a matricellular protein and plays a vital role in tissue regeneration, fibrosis and wound healing. However, data about its significance in nephrology are limited. We investigated the correlation between urinary periostin excretion and its clinical significance including renal histologic findings and prognosis in IgA nephropathy (IgAN). Of 399 patients from a glomerulonephritis cohort recruited between January 2009 and December 2014, 314 were enrolled. Serum and urine periostin (uPOSTN) were measured using enzyme-linked immunosorbent assay. We divided the patients into 3 groups by uPOSTN/creatinine (uPOSTN/Cr): group 1 (undetectable), group 2 (lower than the median) and group 3 (higher than the median). The uPOSTN level was correlated with pathologic classifications and both initial and final IDMS-MDRD estimated glomerular filtration rates (eGFRs; p < 0.001). Histologically, group 3 patients were correlated with severe interstitial fibrosis/tubular atrophy (p = 0.004), interstitial inflammation (p = 0.007), hyaline arteriolosclerosis (p = 0.001) and glomerular sclerosis (p < 0.001). A higher initial uPOSTN/Cr level was associated with a greater decline in eGFR during follow-up (p = 0.043 when initial eGFR ≥60; p = 0.025 when eGFR <60 ml/min/1.73 m2), and the renal outcomes with end-stage renal disease (ESRD; p = 0.003), ESRD and/or eGFR decrease of >30% (p = 0.033) and ESRD and/or eGFR decrease of >50% (p = 0.046) occurred significantly more in group 3. In multivariate analysis, uPOSTN group 3 (hazards ratio 2.839, 95% CI 1.013-7.957; p = 0.047) was independently associated with ESRD in IgAN patients. uPOSTN/Cr value at initial diagnosis correlated with renal fibrosis and predicted the renal outcomes in patients with IgAN. It could be a promising urinary biomarker for renal fibrosis.