The authors report a Br doping effect on the NO2 gas sensing properties of a two-dimensional (2D) SnSe2 semiconductor. Single crystalline 2D SnSe2 samples with different Br contents are grown by a simple melt-solidification method. By analyzing the structural, vibrational as well as electrical properties, it can be confirmed that the Br impurity substitutes on the Se-site in SnSe2 serving as an efficient electron donor. When we measure the change of resistance under a 20 ppm NO2 gas flow condition at room temperature, both responsivity and response time are drastically improved by Br doping from 1.02% and 23 s to 3.38% and 15 s, respectively. From these results, it can be concluded that Br doping plays a key role for encouraging the charge transfer efficiency from the SnSe2 surface to the NO2 molecule by elaborating Fermi level in 2D SnSe2.