We investigated the structural and magnetic properties of single-crystalline SmCuAs₂ using high-resolution synchrotron X-ray diffraction and X-ray resonant magnetic scattering (XRMS) at the Sm L₂ and L₃ edges. Temperature-dependent diffraction measurements confirm that SmCuAs₂ maintains its tetragonal symmetry from room temperature down to 8 K, with lattice parameters showing anomalous behavior below the resistivity minimum (T ≈ 30 K). Notably, the c-axis lattice parameter exhibits a plateau and subsequent increase near the Néel temperature, indicating magnetoelastic coupling. XRMS measurements reveal a commensurate antiferromagnetic structure with a propagation vector q = (0, 0, 0.5). Our measurement shows that the Sm moments are aligned within the ab plane and arranged in a ++-- stacking along the c-axis. Comparison with related RECuAs₂ compounds (RE = Pr, Nd, and Gd) suggests that in-plane moment orientation and associated magnetic frustration play a key role in the emergence of the resistivity minimum. Differences in spin-orbit and magnetoelastic coupling across the series highlight their importance in governing low-temperature transport behavior.