<p>Structural, optical, and magnetic properties of La_0.6Sr_0.4MnO₃ and La_0.3A_0.3Sr_0.4MnO₃ (A = Gd, Tb, Dy, Ho, Er) perovskite nanoparticles synthesized using the sol–gel citrate–nitrate method were investigated. The XRD analysis of La_0.3A_0.3Sr_0.4MnO₃ nanoparticles revealed a structural phase transition from rhombohedral to monoclinic with decreasing ionic radius of the rare earth substitution atoms. Crystallite size, calculated using the Scherrer method, decreased with smaller rare earth ionic radii, highlighting the impact of electronegativity on crystallite size. Raman spectroscopy highlighted structural disorder induced by rare earth doping, while field-emission scanning electron microscopy and EDX confirmed particle size reduction and homogeneous substitution. UV–Vis analysis demonstrated that the rare earth substitution reduces the bandgap of La_0.6Sr_0.4MnO₃ due to lattice distortions. This ability to control and modify the bandgap energy of these materials presents opportunities for designing tailored materials with desired electronic properties for various optoelectronic applications, such as photovoltaics and sensors. Magnetic studies revealed that saturation magnetization and coercivity decreased with substitution, driven by reduced Mn–O–Mn bond angles and increased magnetic dead layer. Smaller A-site ionic radii and particle size contributed to a weaker Mn³⁺–Mn⁴⁺ double exchange, leading to reduced magnetization.</p>