Altermagnets have garnered great interest due to their nonrelativistic spin splitting and unique physical properties. However, the control of altermagnetic states remains underexplored. Here, we propose a multiferroic state, i.e., ferroelastic altermagnetic state, in which ferroelastic strain couples directly to the spin splitting. Through symmetry analysis and first-principles calculations, we identify the ferroelastic $d$-wave altermagnetism of puckered pentagonal ${\mathrm{CoSe}}_2$ monolayer. Interestingly, uniaxial stress can induce a ferroelastic phase transition, accompanied by a ${90}^{\circ }$ rotation of the spin-splitting bands. Cooperative rotation of the lattice and Néel vectors preserves the sign of Kerr angle, whereas noncooperative rotation reverses it. Our work provides a general strategy for manipulating altermagnetism in multiferroic systems and opens other avenues for exploring emergent magnetoelastic phenomena.

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• Altermagnetism
• Ferroelasticity
• Magneto-optical Kerr effect
• Altermagnets
• Multiferroics
• Transition metal dichalcogenides
• Density functional calculations
• Monte Carlo methods