Fe³⁺ was efficiently detected using a novel turn-on hyperbranched polymeric spectral sensor based on the linear relationship between the absorbance at 562 nm (or the fluorescence intensity at 578 nm) of the sensor–Fe³⁺ solution and the Fe³⁺ concentration, as well as the visible (from colorless to pink) and fluorescence (from dark to orange-red) color changes. The sensor showed great selectivity to Fe³⁺ and the competition ions had negligible influence. Further, the sensor could be applied in real water sample assays with a similar accuracy to that of AAS and it could be facilely recovered using Na₂S.