In this work, a new spectrophotometric method for the determination of Fe(III) ions has been developed. The method is based on ternary complexes formed between green-synthesized silver nanoparticles (AgNPs), a bis-azo reagent (R), and cetyltrimethylammonium bromide (CTAB). AgNPs were synthesized using a starch–glucose system, and the reagent R was synthesized by the reaction of benzidine and pyrogallol. The interaction of Fe(III) with the R, Ag–R, and Ag–R–CTAB systems was studied with respect to pH, reagent concentration, temperature, and time factors. The Ag–R–CTAB system gave maximum absorbance at 487 nm and showed a clear bathochromic shift, and the molar absorptivity was 2.2×10⁴ L·mol⁻¹·cm⁻¹, which is much higher than that of the binary systems. The complex is stable for 24 hours and obeys Beer’s law in the range of 0.10–2.24 mg/L. Common ions do not interfere even at high concentrations. The method was successfully applied to fruit samples and showed good accuracy and repeatability. The results show that azo compounds modified with AgNPs significantly increase the sensitivity of Fe(III) determination.