Two-dimensional multiferroics with out-of-plane ferroelectricity hold significant promise for miniaturized magnetoelectric spin-orbit transistors, yet systems combining robust ferroelectricity and strong magnetoelectric coupling are exceedingly rare. Here, we demonstrate that epitaxial strain stabilizes out-of-plane ferroelectricity in exfoliated two-dimensional Ruddlesden-Popper derivatives. The hybrid improper ferroelectric $Pc$ phase transitions to a competing $P{2}_1$ phase with purely in-plane polarization upon switching, accompanied by a ${90}^{\circ }$ rotation of weak ferromagnetism. Crucially, the $Pc$ phase exhibits altermagnetism, while $P{2}_1$ displays full-Brillouin-zone band splitting, with persistent spin textures rotating ${90}^{\circ }$ at the phase boundary. This work establishes a pathway to engineer two-dimensional multiferroics that integrate vertical polarization, magnetoelectric coupling, and switchable spin textures—key features for next-generation spintronic devices.

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• Electronic structure
• Ferroelectricity
• Magnetism
• Multiferroics
• First-principles calculations
• k dot p method