A highly stable ternary complex of iron(III) with 5-(4-nitrophenylazo) salicylic acid (NPAS) and Eosin (Es) in acidic medium at room temperature gave a maximum absorption at 545 nm with a molar absorptivity 2.81×104 L mol−1 cm−1. A spectrophotometric method using these ligands was developed and optimized in terms of pH, stability of the complex, amount of reagent required, sensitivity, linearity, and the effect of various foreign ions was studied. The linear range for iron(III) determination is 0.18–6.0 mg L−1. The method was sensitive, accurate, and all the reagents were stable under the working conditions. Moreover, the method was easy to perform for the determination of iron in pharmaceutical and water samples.