There has been substantial concern over the potential for plastic particles to serve as vectors for toxic heavy metals through adsorption. The mechanisms involved in this process are particularly understudied in the context of nano-sized particles. To address this gap, we report the adsorption of Cu2+ ions onto polystyrene nanoparticles (PS-NPs) in simulated aquatic environmental conditions, with and without UV-ageing. Surfactant-free PS-NPs were synthesised with average diameters of 130, 260 and 520 nm. Scanning electron microscopy and dynamic light scattering characterisation of PS-NPs revealed increased instability and aggregation with decreasing particle size, following UV-ageing. While PS-130 nm initially exhibited higher Cu2+ adsorption capacity in DI water, this value shifted in favour of PS-260 nm after UV-ageing, due to higher colloidal instability of PS-130 nm. Yet, in seawater, 3-4 times reduction in adsorption was observed for all the samples, compared to DI water, alongside accelerated equilibrium times, attributed to the competition of ions in seawater. On the surface of UV-aged PS-NPs, X-ray photoelectron spectroscopy analysis showed an increase in Cu(OH)2 bonds after adsorption, resulting in increased activation energy of adsorption compared to original samples. Adsorption isotherms favoured the Freundlich model, highlighting a distinct isotherm shape for each medium and a tendency toward a linear isotherm as the particle size and/or temperature increased. Thermodynamic calculations showed the adsorption interaction was exothermic, in addition to the higher spontaneity of adsorption by smaller particles. This research enhances our understanding of PS-NPs behaviour in aquatic environments, paving the way for exploring metal ions’ interaction with complex plastic nanoparticles in future studies.