Two-dimensional (2D) multiferroic materials with the coexistence of spin and electric polarizations are appealing because of their rich fundamental physics and promising technological applications. However, up to now, 2D ferromagnetic (FM) ferroelectric (FE) multiferroic semiconductors are still rather scarce and possess very low Curie temperature ($T_{\mathrm{C}}$). Here, we propose that room-temperature ferromagnetism and electric polarization could exist in a 2D penta-lattice. Based on first-principles calculations, we predict that the penta-CrPS monolayer is a stable room-temperature polar FM semiconductor with $T_{\mathrm{C}}$ up to $\sim 560$ K. The strong FM super-exchange between adjacent Cr ions originates from the small splitting of occupied and empty Cr-3d orbitals induced by the square planar crystal field. The in-plane 4-fold electric polarization can be characterized by the rotation of P-S pairs with a moderate electric polarization transition barrier of $\sim 0.650$ eV. Similar high-temperature ferromagnetism and electric polarization have also been observed in other 2D penta-lattices, e.g., penta-MoPS and penta-RePS monolayers, whose $T_{\mathrm{C}}$ are $\sim 1060$ and $\sim 700$ K, respectively. These findings provide a new direction and potential candidates for realizing room-temperature 2D polar FM semiconductors, which will significantly motivate the research progress of spintronics.

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• First-principles calculations
• Magnetism
• 2-dimensional systems
• Multiferroics
• Semiconductors
• Crystal structures
• Density functional theory