Aluminium ions play an important role in various biological processes and in the development of methods for its detection has received significant attention recently. Among the reported analytical methods for aluminium ions, fluorescence based detection is considered very advantageous because of its extreme sensitivity and simplicity. However, a large majority of these reported sensor systems for aluminium detection are either based on synthetically involved fluoregenic probes or perform sensing operation via single wavelength based response which makes them susceptible to error due to minute variations in analyte-independent experimental variables. Thus, in the present contribution, we describe a fluorescence ratiometric sensor for the sensitive and selective detection of Al³⁺ which is based on Al³⁺ ion dependent modulation of aggregate-monomer equilibrium of Thioflavin-T-Polyacrylic acid system. The molecular rotor dye, Thioflavin-T (ThT), undergoes efficient aggregation, with a distinct emission band, in the presence of a carboxyl laced polyelectrolyte, poly (acrylic acid), PAA. This ThT-PAA monomer-aggregate system undergoes significant modulation in the presence of Al³⁺ ion due to selective interaction of hard Al³⁺ ions with hard donors like oxygen containing carboxylate groups. This modulation provides a fluorescence ratiometric response for Al³⁺ originating from distinct spectroscopic features of ThT monomers and aggregates. Overall, the sensor system provides a selective and sensitive response towards detection of Al³⁺ with a LOD of 0.9 μM. Importantly, the sensor system is composed of commercially available components and provides freedom from the reliance on time-consuming and tedious protocols required for producing the probe molecules. The ratiometric response provides a robustness to the analytical performance as compared to the system that operates through single wavelength based measurements. We have also demonstrated the sensor system response in real water samples from tap water.