Abstract <p>The structural, electronic, and optical properties of halide perovskite Cs₃Bi₂Br₉ have been investigated via density functional theory. Using a generalized gradient approximation by Perdew, Bruke, and Ernzerhof exchange-correlation function, the calculated lattice parameters and electronic properties at ambient pressure are in good agreement with the other calculations and experimental data. The calculated pressure dependence of lattice parameters <i>a</i> (<i>b</i>) and <i>c</i> were studied, and both of them decrease with increasing pressure in the pressure ranging from 0 to 20 GPa. These calculated results indicate that the linear compressibility along <i>c</i> axis is significantly higher than <i>a</i> and <i>b</i> axes, which shows that the intermolecular bonding along the <i>c</i> axis is softer and hence easily compressible than other crystallographic axes. The electronic structure shows the top of the valence band and bottom of the conduction band minima are, unusually, dominated by Bi s and p states, respectively. According to our calculation, the band gap decreases with increasing pressure. Moreover, optical constants including dielectric constant, absorption coefficient, reflectivity, refractive index and the extinction coefficient of Cs₃Bi₂Br₉ are analyzed.</p>