Discovery of new two-dimensional (2D) materials not only has important fundamental research significance but also has promising application prospects. Through systematic structure searching combined with density functional theory calculations and simulations, we predict a new 2D monolayer HgS2 material featuring with a previously unreported type of pentagonal structure with orthorhombic P21212 symmetry. The penta-HgS2 monolayer shows good thermal, thermodynamic, mechanical and dynamic stabilities, exhibiting ferroelastic phase transition with an ultralow switching energy barrier (about 1 meV/atom), high ultimate strength of 12 N/m, moderate indirect band gap of 2.25 eV. In addition, the penta-HgS2 monolayer shows significantly anisotropic carrier mobility with a large anisotropy ratio of 98.5 along the y direction. More importantly, the penta-HgS2 monolayer exhibits abundant catalytic active sites, low Gibbs free energy (ΔGH ranging from 0.656 to 1.687 eV) for hydrogen evolution reactions, excellent solar light absorption capacity (∼105 cm−1) with the solar-to-hydrogen efficiency reaching up to 11.83%. The excellent ultimate tensile ability and the ultralow ferroelastic phase transition energy barrier implies penta-HgS2 monolayer material can be used to manufacture flexible mechanical and nanoelectronic devices. The suitable band edge alignments, ultrahigh carrier mobility anisotropy, high catalytic activity and excellent visible light adsorption performance indicate that penta-HgS2 monolayer is a promising candidate photocatalyst for high-performance photocatalytic water splitting. The present work not only supplements a new member to the 2D pentagonal structure material family, but also points out its promising applications in nanoscale devices and photocatalytic hydrogen productions.