Two-dimensional multiferroics promise low-power, multifunctional devices, yet the intrinsic coexistence and mutual control of three coupled ferroic orders in a single layer remains elusive. Here, we identify pentagonal monolayer $\mathrm{Fe}{\mathrm{O}}_2$ as an intrinsic triferroic altermagnet where ferroelectric (FE), ferroelastic (FA), and altermagnetic (AM) orders coexist and are tightly coupled, accompanied by a competing antiferroelectric (AFE) phase using first-principles calculations. The sole presence of glide mirror ${\mathrm{M}}_x$ symmetry in a $\mathrm{Fe}{\mathrm{O}}_2$ sublayer, with the breaking of fourfold rotation ${\mathrm{C}}_{4z}$ symmetry, induces in-plane vector ferroelectricity and twin-related ferroelastic strains. Both FE and AFE phases break combined parity-time symmetry and display sizable altermagnetic spin splitting with Néel temperatures over 200 K. Electric-field induced rotation of the FE polarization reverses the sign of the spin splitting, while in-plane uniaxial strain triggers ferroelastic switching that simultaneously rotates the FE polarization vector by 90° and reverses the AM state. These electric-field- and strain-mediated pathways interlink six distinct polarization states that can be selected purely by electric fields and/or mechanical strain. This work extends intrinsic triferroicity to pentagonal monolayers and outlines a symmetry-based route toward mechanically and electrically configurable altermagnetic spintronics.

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• Ferroelasticity
• Ferroelectricity
• 2-dimensional systems
• Altermagnets
• Multiferroics
• Density functional calculations