Background: Recurrence is common after nephrectomy for renal cell carcinoma (RCC), but no circulating biomarkers are available to identify patients at highest risk of recurrence who may benefit from adjuvant therapy. Kidney injury molecule-1 (KIM-1) is overexpressed in RCC and its ectodomain circulates in plasma. We investigated whether plasma KIM-1 is a prognostic biomarker in patients with localized RCC after nephrectomy. Methods: Banked plasma samples were analyzed from the ECOG-ACRIN 2805 (ASSURE) trial evaluating adjuvant sunitinib, sorafenib, and placebo in resected high-risk RCC. KIM-1 levels were measured at trial enrollment 4-12 weeks post-nephrectomy (baseline) and on cycle 2 day 1 (C2D1) using a previously validated microbead assay. A lognormal accelerated failure time model was used to test for association between circulating KIM-1 and disease-free survival (DFS). Results: Plasma samples from 418 patients were analyzed. In univariable and multivariable analyses, higher post-nephrectomy KIM-1 was associated with worse DFS across all study arms. This association remained independently significant after adjustment for Fuhrman grade, T-stage, N-stage, and tumor histology (survival time ratio 0.56 for 75^th vs 25^th percentile of KIM-1, 95% CI 0.42-0.73, p < 0.001). The association between KIM-1 and DFS was stronger among patients with pathologic nodal involvement. The addition of baseline KIM-1 improved the concordance of both the SSIGN and UISS prognostic models (SSIGN concordance 0.57 vs 0.43, p = 0.05; UISS concordance 0.60 vs 0.40, p = 0.0005). C2D1 KIM-1 was not an independent predictor for DFS after adjusting for baseline KIM-1. Conclusions: Elevated plasma KIM-1 level at post-nephrectomy baseline is associated with worse DFS in RCC. This is consistent with the hypothesis that post-nephrectomy plasma KIM-1 may be a biomarker for microscopic residual disease. The model was additionally adjusted for papillary and chromophobe histology, sex, and ECOG performance status.