LaSi3N5-based phosphor doped with Sm was prepared by the nitridation of LaSi-Si-Si3N4-Sm2O3 powder mixture. The emission spectrum shows two main bands with maxima at 595 nm in the orange region and at similar to 650 nm in the red region. The excitation spectrum of Sm-doped LaSi3N5 shows a maxima at 585, 570, and 405 nm. First-principles density-functional theory calculations were performed using Vienna ab initio simulation package to enhance the understanding of the electronic structure of the stoichiometric LaSi3N5 and Sm-doped LaSi3N5. The electronic structure and band gaps were calculated in 2 x 1 x 2 supercell with 144 atoms using the more precise screened Coulomb hybrid functional HSE06. Both La3+/Sm3+ and La3+/Sm2+ substitutions were calculated. The calculated band gap of Sm(III)-doped LaSi3N5 is 2.01 eV, in reasonable agreement with the experimental value of 2.12 eV, but corresponds to the unrealistic transition between the N, Si p states, and unoccupied Sm 4f states. The band gap of 1.43 eV calculated for Sm(II)-doped LaSi3N5 is smaller than the available experimental value, but corresponds to the correct transition between nonbonding Sm 4f states and empty La 5d states. Optical properties are found to be governed by f electrons of the Sm(II) dopant.