This study uses modified silver nanoparticles (AgNPs) and Tollens' reagent to explore a novel colorimetric sensor for formaldehyde detection. AgNPs was synthesized using sodium citrate and tannic acid. Characterization of AgNPs using UV–Vis spectroscopy, XRD, and TEM were observed, confirming their size and structure. AgNPs was modified with Tollens' reagent and investigated the sensor's response to various formaldehyde concentrations. The results showed a linear relationship between the maximum absorbance wavelength and formaldehyde concentration, demonstrating the sensor's potential for quantitative detection. Crucially, the AgNPs@Tollens system exhibited remarkable selectivity towards formaldehyde over other aldehydes such as acetaldehyde and benzaldehyde, as evidenced by distinct UV–Vis absorption spectra; only formaldehyde induced a strong surface plasmon resonance (SPR) peak at approximately 420 nm, while other aldehydes showed negligible. TEM analysis validated the rise in AgNPs size upon exposure to formaldehyde. Zeta potential measurements demonstrate a significant decrease in colloidal stability upon the addition of formaldehyde, as indicated by the shift to a less negative (−21.4 mV) zeta potential for AgNPs@Tollens with formaldehyde. This reduced electrostatic repulsion explains the increased tendency for agglomeration, which is consistent with observations from TEM analysis. The limit of detection (LOD) for formaldehyde was determined to be 11.20 μM, indicating the sensor's high sensitivity. The sensor demonstrates selectivity for formaldehyde compared to other tested interferents. This novel colorimetric sensor offers a method that is not only straightforward but also quick and economical for formaldehyde detection in various environments. © 2025 The Authors