An optically pumped mid-infrared HBr gas laser based on hollow-core fiber (HCF) is reported. A tunable narrow linewidth 2 μm continuous-wave thulium-doped fiber amplifier is used to pump a 4.4 m anti-resonance hollow-core fiber filled with low-pressure HBr gas. When the seed wavelength is precisely tuned to the R(2) absorption line near 1971.7 nm of hydrogen bromide isotope H ⁷⁹Br, the H ⁷⁹Br molecules in the vibrational ground state v₀ will transit to the vibrational excited-state v₂, then a population inversion will be formed between the vibrational excited states v₂ and v₁. Due to the transition selection rules, two transitions will simultaneously occur by the spectral lines R(2) and P(4), whose wavelengths are 3977.2 nm and 4165.3 nm, respectively. When the HBr pressure is 6.2 mbar, the maximum output power of 4 μm laser is 125 mW, and the optic-to-optic conversion efficiency is about 10% in terms of the pump light coupled into the HCF. By further improving the transmission loss spectrum of the HCF and increasing the coupling efficiency of the pump light, the laser efficiency and output power can be greatly improved, and a wide-range tunable mid-infrared laser emission could be obtained owing to the energy level characteristics of HBr molecules in the future.