Two-dimensional materials have enabled unprecedented control over material properties through interlayer engineering, where mechanisms such as sliding can directly modify symmetry configurations and functionalities. Based on first-principles calculations, we demonstrate that bilayer $\mathrm{VP}{X}_3$ $(X=\mathrm{S},\mathrm{Se},\mathrm{Te})$ provides an ideal platform in which interlayer sliding simultaneously activates and couples sliding ferroelectricity (FE) and altermagnetism (AM). Here, a switchable out-of-plane FE polarization emerges from the sliding-induced breaking of inversion symmetry, whereas momentum-dependent spin splitting arises from crystal symmetries, without spin-orbit coupling. Crucially, the interlayer sliding displacement that reverses FE polarization also fully inverts the spin polarization in the AMs, equivalent to a 180° spin reversal. Remarkably, this magnetoelectric (ME) coupling is directly manifested as clear sign reversals in the magneto-optical Kerr rotation. Furthermore, interlayer compression enhances both the FE polarization and AM band splitting in a nearly linear manner, demonstrating strain-tunable multiferroic behavior. Our results establish $\mathrm{VP}{X}_3$ bilayers as a prototypical system for achieving deterministic control over coupled ferroic orders via atomic-scale layer manipulation, opening avenues for designing spintronic and ME devices.

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• Ferroelectricity
• Magnetoelectric effect
• Multiferroics