Based on first-principles spin-polarized density functional theory (DFT) calculations, the geometric, electronic structures, and magnetic properties of semihydrogenated gallium nitride (GaN) sheet are investigated. The HGaN conformation exhibits ferromagnetic (FM) ground state with a magnetic moment of about 1.0 µB per unit cell. The ferromagnetism is mainly attributed to the decrease in the charge transfer from Ga to N atoms after hydrogenated, which leads to the partial occupancy of the N-2pz orbitals. The half-boat conformation is predicted to be the most stable structure with an indirect bandgap of 2.34 eV. While the GaNH conformation shows metallic behavior without magnetism.