We explore the structural, electronic and magnetic properties of the monolayer SnS2 systems doped with 5d transition metals (TM) atoms X (X = W, Re, Os, Ir, Pt, Au or Hg) using first-principles approach. The formation energies show that these materials can be easily fabricated under the S-rich experimental condition. The spin-polarized phases are more favorable than non spin-polarized phases. The pristine monolayer SnS2 is a nonmagnetic semiconductor with an indirect band gap of 1.579 eV, the 5d TM atoms X (X = W, Re, Os, Ir, Pt, Au or Hg) doped monolayer SnS2 systems have the total magnetic moments of 2, 3, 2, 1, 0, 1, 2 µB, respectively, because the octahedral coordinated six S ligands bring in a larger crystal field splitting than exchange splitting onto the 5d TM atoms X. Furthermore, the spin-polarized band structures show that the W, Re or Hg-doped monolayer SnS2 systems are magnetic semiconductors, but the Os, Ir or Au-doped monolayer SnS2 systems are magnetic half-metallic with the wider band gaps of 1.509, 1.632 or 1.383 eV respectively in one of the two spin channels, and thus are useable in spintronic fields.