Turbulent boundary layers (TBLs) subjected to adverse pressure gradients (APGs) are highly relevant to industrial aerodynamic applications, yet the effect of freestream turbulence (FST) on TBLs developing under moderate APGs remains insufficiently understood. To address this, we conducted wind tunnel experiments to investigate the effects of FST on a developing TBL over a NACA 0015 airfoil. Varying the angle of attack (2 and 4${}^{\circ }$) adjusted the pressure gradient, and hotwire anemometry measured boundary layer properties at different chordwise positions ($x/c$ = 0.400–0.625, with $\beta =({\delta }^{*}/{\tau }_w)d{P}_e/dx$ = 0.2–1.5). The FST level was increased using static grids, resulting in levels ranging from 0.15 to 6$\%$. The chord-based Reynolds number was kept constant at around 250,000 for all configurations. The results show that increasing FST systematically modifies the mean-flow development and turbulence structure of the APG boundary layer. Higher FST levels reduce the shape factor and partially suppress the APG-induced wake in the mean velocity profile, while increasing the skin-friction coefficient towards values closer to canonical ZPG behaviour. The streamwise velocity variance is strongly amplified in the outer region, while spectral analysis shows that energetic FST-associated large-scale motions, with characteristic wavelengths of order ${\lambda }_x/\delta \cong 13$, penetrate into the boundary layer and contribute to the near-wall variance. Their influence close to the wall becomes more pronounced as the adverse pressure gradient increases. The findings demonstrate that FST is a governing parameter in spatially developing APG TBLs over airfoils, since its influence appears to be amplified by the adverse pressure gradient, and must therefore be accounted for when interpreting mean-flow development and turbulence statistics in realistic aerodynamic environments.