<p>The multiaxial deformation behavior and yield-locus evolution of the non-heat-treatable Al–Mg alloy AA5052 were systematically investigated using uniaxial and planar biaxial tensile tests conducted under both load-controlled and displacement-controlled conditions. Planar biaxial tests were performed using optimized cruciform specimens, enabling precise control of stress and strain paths and full-field strain measurement via digital image correlation. Load-controlled biaxial loading revealed pronounced stress serrations and band-type strain localization, indicating Portevin–Le Châtelier–type plastic instabilities characteristic of Al–Mg alloys. Yield loci constructed using the plastic work equivalence approach exhibited progressive, non-uniform expansion with increasing plastic strain, with a more pronounced evolution toward the biaxial stress region compared to uniaxial loading. Displacement-controlled tests demonstrated a strong dependence of flow stress and yield-surface evolution on kinematic constraints. Furthermore, strain-path-change experiments highlighted transient stress states deviating from monotonic yield loci due to redistribution of plastic work and deformation history. The results underscore the necessity of biaxial, path-dependent experimental data for reliable constitutive modeling and forming-process design of AA5052 sheets.</p>