We report for the first time a technique in the detection of chromium(VI) [Cr(VI)] ions using ultraviolet-visible (UV-Vis) spectrophotometer that involves silver nanoparticles (AgNPs) capped with formaldehyde-chitosan as the sensor. Here, chitosan cross-linked with formaldehyde produced an extra stability from the aspect of signal stability of the sensor. This study aimed to synthesise and characterise formaldehyde-chitosan stabilised AgNPs (formaldehyde-chitosan@AgNPs) and apply them as Cr(VI) ion sensors. The stability and linearity of the signal of the formaldehyde-chitosan@ AgNPs were compared with the signal of AgNPs according to their ability as Cr(VI) ion sensors based on the UV-Vis spectroscopy. The AgNPs were synthesised by the chemical reduction method using 1% sodium citrate (10:1). Furthermore, 3.3% chitosan was added, which had been cross-linked with 2.5% formaldehyde in a ratio of 1:1 (v:v). The characterisation results showed that the synthesised materials have a peak absorption at a wavelength of 416 nm and have functional groups of -OH and -C=O as the characteristics of chitosan and formaldehyde. The formaldehyde-chitosan@AgNPs had square or rectangular structures with a size distribution of less than 1000 nm. The formaldehyde-chitosan@AgNPs produced a more stable signal and had better linearity than AgNPs at a wavelength of 490 nm. Moreover, formaldehyde-chitosan@AgNPs had a precision value of 0.15% and a linearity level of 0.9914 in the concentration range of 1 ppm–100 ppm, with limit of detection (LOD) and limit of quantisation (LOQ) values of 0.329 ppm and 0.751 ppm, respectively.