The CrBr₃ monolayer is a two-dimensional semiconductor material with intrinsic ferromagnetism. However, the low Curie temperature of CrBr₃ monolayer limits its practical development in innovative spintronic devices. The electronic and magnetic properties of transition-metal atoms doped CrBr₃ monolayer have been systematically investigated by using the density functional theory calculations. The formation energy elucidates that all 3d transition metal (TM) atoms prefer to be doped in the middle of a hexagon (H) site of CrBr₃ monolayer. And all the TM atoms, except the Zn atom, can bond strongly to the surrounding Cr atoms with sizable formation energy. The results also indicate that the magnetic moment of TM-CrBr₃ system changes as a result of the charge transfer between TM atom and adjacent Cr atom. In addition, comparing with the intrinsic CrBr₃, the T_C of TM-CrBr₃ system increases significantly, which means that the ferromagnetic stability of CrBr₃ monolayer is enhanced. In particular, the T_C of CrBr₃ with Sc atom can be increased by 159%. The enhancement of ferromagnetism is mainly due to the competition between the direct exchange and the superexchange interaction. We also find that the electronic properties of the TM-CrBr₃ systems are diverse. For example, Sc-, Ti-, V-, Mn-, Fe-, Co-, Ni-, Cu- and Zn-CrBr₃ exhibit spin gapless semiconductor (SGS) properties with 100% spin polarization at Fermi level. The TM-CrBr₃ system can be adjusted from semiconductor to half-metal when Cr atoms are doped into the CrBr₃ monolayer. This work, together with recent achievements in the field of two-dimensional ferromagnetic materials, provides an experimentally achievable guide for realizing the preparation of TM-CrBr₃ system with high Curie temperature. Moreover, the possibility of application of these systems in nanoelectronics and spintronics is increased.