Environmental pollution by the heavy metal lead (Pb2+) poses significant health risks, including kidney damage and neurotoxicity in children. Gold nanoparticles (AuNPs) have shown promise as colorimetric sensors for visually detecting Pb2+ through surface plasmon resonance. This study developed a colorimetric method using thioglycolic acid (TGA) as a conjugate, leveraging its strong S-Au bond and carboxyl group to enhance AuNPs stability and Pb2+ specificity. The method was optimized and examined using UV-visible spectrophotometry, High-Resolution Transmission Electron Microscopy (HRTEM), and Fourier Transform Infrared Spectroscopy (FTIR). Optimal conditions were identified as 700 μL AuNPs, 500 μM thioglycolic acid, and pH 10.0 for 10 minutes. The synthesized TGA-AuNPs could detect Pb2+ at a limit of 9.5 μg mL−1. The sensor demonstrated specificity to Pb2+ against Ba2+, Mn2+, Cu2+, Mg2+, and Hg2+. The application to water samples from Lake Kenanga, Puspa, and FMIPA in Universitas Indonesia indicated that Pb2+ levels were below the detectable concentration. This research successfully developed a simple, fast, cost-effective TGA-AuNPs colorimetric sensor for real-time Pb2+ detection in water. © 2025 The Royal Society of Chemistry.