Background: Even today, a non-invasive biomarker to identify donors with enhanced risk for renal impairment is missing. Dickkopf 3 (DKK3) can cause tubulointerstitial fibrosis and is associated with rapid eGFR loss. The aim of our work was to analyze whether DKK3 can be used as a non-invasive alert marker for an increased risk of loss of kidney function in living kidney donors (LKDs). Methods: All donors who were examined between July 2022 and June 2023 (n = 117) were included. DKK3 was measured in the urine. The collected patient-related data were compared with parameters before donation. The study cohort was stratified by DKK3 values (</≥200). Results: In the follow-up, 89 donors had a DKK3 value < 200 (group 1) and 28 donors had a DKK3 value ≥200 (group 2). During post-donation follow-up, renal function in group 1 was significantly better than that in group 2 (p = 0.01), although no difference in renal function before donation was detected (p = 0.84). Group 2 showed also a greater eGFR loss over time than group 1. Conclusions: LKDs with elevated DKK3 levels in the FU had impaired kidney function without evidence of increased risk factors pre-donation. DKK3 can represent a possible monitoring tool for kidney function in LKDs.