An experimental and theoretical study is reported to investigate the influence of bromine doping on CH 3 NH 3 Pb(I 1– x Br x ) 3 perovskite for Br compositions ranging from x = 0 to x = 0.1, in which the material remains in the tetragonal phase. The experimental band gap is deduced from UV–vis absorption spectroscopy and displays a linear behavior as a function of bromine concentration. Density functional theory calculations are performed for five different series of randomly doped structures in order to simulate the disorder in bromine doping sites. The computations predict a linear variation of the lattice parameters, supercell volume, density, band gap, and formation energy in the considered doping range. The calculated evolution of the band gap as the function of Br doping is in excellent agreement with the experimental data, provided that different Br doping configurations are included in the simulations. The analysis of the structural and electronic properties shows a correlation between the increase of the band gap and the increased distortion of the Pb(I 1– x Br x ) 6 octahedrons. Additionally, the simulations suggest that in CH 3 NH 3 Pb(I 1– x Br x ) 3 bromine doping is likely to occur at both the equatorial and apical positions of the octahedrons.