Updated project metadata. Purpose: Renal cell carcinoma (RCC) is often diagnosed incidentally as an early-stage small renal mass (SRM; pT1a, ≤ 4 cm). Increasing concerns surrounding the overtreatment of patients with benign or clinically indolent tumors has led to a shift in current treatment recommendations, especially for elderly and infirm patients. There are currently no available biomarkers to accurately stratify patients according to risk. Therefore, we set out to identify early biomarkers of RCC progression. Experimental Design: We employed label-free LC-MS/MS and targeted parallel-reaction monitoring (PRM) to identify early, non-invasive diagnostic and prognostic biomarkers for early-stage RCC-SRMs. In total, we evaluated 115 urine samples, including 33 renal oncocytoma (≤ 4 cm) cases, 30 progressive and 26 non-progressive clear cell RCC-SRM (ccRCC-SRM) cases, in addition to 26 healthy controls. Results: We identified nine endogenous peptides which displayed significantly elevated expression in ccRCC-SRMs relative to healthy controls. Peptides NVINGGSHAGNKLAMQEF, VNVDEVGGEALGRL, and VVAGVANALAHKYH showed significantly elevated expression in ccRCC-SRMs relative to renal oncocytoma. Additionally, peptides SHTSDSDVPSGVTEVVVKL and IVDNNILFLGKVNRP displayed significantly elevated expression in progressive relative to non-progressive ccRCC-SRMs. Peptide SHTSDSDVPSGVTEVVVKL showed the most significant discriminatory ability (AUC: 0.76, 95% CI: 0.62 to 0.90, p = 0.0027). Patients with elevated SHTSDSDVPSGVTEVVVKL expression had significantly shorter overall survival (HR: 4.13, 95% CI: 1.09 to 15.65, p = 0.024) compared to patients with lower expression. Conclusions: Our in-depth peptidomic analysis identified novel biomarkers for early-stage RCC-SRMs. Characterization of urinary peptides may provide insight into early RCC progression and could potentially help assign patients to appropriate management programs.