Two dimensional atomic crystals with pentagonal building blocks have attracted extensive interest in recent years for their fundamental significance and potential applications in nanoscale devices. Here, with the help of ab initio calculations based on density functional theory, we report a unique pentagonal structured NiS2 monolayer in P4[combining macron]21m symmetry, named P-NiS2. Its dynamic stability has been confirmed by phonon mode analysis. Molecular dynamics simulations and total-energy calculations show that this new P-NiS2 has robust thermal stability and energetically more stable than all other reported NiS2 monolayer structures. Electronic band structure calculations show that it is a semiconductor with an indirect band gap of 1.94 eV. Furthermore, we find that small strain triggers a transition from the indirect to direct band gap for this P-NiS2, suggesting its great potential for applications based on strain-engineering techniques.