Lanthanum cuprate (La₂CuO₄) perovskite-type nanoparticles were synthesized by facile microwave-assisted combustion (2.45 GHz/900 W) for 15 min and calcinated at 500, 700, 900, and 1000 °C for 2 h. The effect of calcination temperature on the structural, optical, magnetic, and dielectric properties was investigated. The XRD studies confirmed that the perovskite nanoparticles with different temperatures from 500 to 900 °C possess a single-phase orthorhombic crystal structure of La₂CuO₄. In contrast, when the temperature increased to 1000 °C, the structure changed from orthorhombic to tetragonal. The average crystallite size of the orthorhombic phase is in the range of 37–47 nm. The presence of tensile strain in La₂CuO₄ was determined from Williamson–Hall (W–H) analysis. The appearance of FT-IR bands at approximately 634 and 970 cm⁻¹ was correlated to the La–O and Cu–O stretching modes of the orthorhombic La₂CuO₄ phase. UV–Vis spectroscopy indicates that calcination temperature over the 500–1000 °C temperature range causes the band gap to decrease from 2.15 to 1.63 eV. The lanthanum cuprate system showed the formation of nanosized crystallized grains with pores and pore walls due to fused grains. The Magnetization–Field (M–H) hysteresis curves revealed the appearance of ferromagnetic behavior at room temperature. The dielectric properties of the fabricated La₂CuO₄ perovskite nanoparticles were evaluated at different temperatures and frequency-dependent dielectric constant, dielectric loss and AC conductivity, respectively.