Creatinine is an essential biomarker for early kidney dysfunction, yet many existing assays are slow and instrument-dependent. Here, we present a rapid colorimetric method for creatinine quantification using tartrate-capped silver nanoparticles (Tr-AgNPs) and UV–Vis spectrophotometry. Tr-AgNPs were synthesized via chemical reduction and confirmed by UV–Vis, FTIR, and TEM. Creatinine induced a decrease in the 394 nm SPR band and the appearance of a 648 nm peak, producing a visible yellow-to-grey color change within one minute. The extinction ratio (A648/A394) enabled sensitive quantification, showing good linearity (R2 = 0.997) and a 68.7 nM detection limit under optimized conditions. DFT results supported strong creatinine–tartrate interactions on the nanoparticle surface. This fast and low-cost Tr-AgNP assay is suitable for point-of-care detection of creatinine in urine. © 2026 Elsevier B.V.