Two-dimensional (2D) altermagnetism has attracted considerable attention due to its low-dimensional character and intrinsic combination of spin splitting with zero net magnetization. In previous work, we developed a general stacking theory (GST) to identify stacking-induced altermagnetism in 2D $A$-type bilayers; however, many systems are symmetry-restricted, preventing realization through stacking alone. Here, we show that Floquet engineering can lift these symmetry constraints by breaking the combined inversion and time-reversal symmetry ($\mathcal{IT}$), enabling a transition from an $A$-type antiferromagnet to an altermagnet in 2D bilayers. By combining the GST with a comprehensive group-theoretical symmetry analysis, we find that only bilayers with initial centrosymmetric point groups $D_{3d}$ or $C_{2h}$ can undergo such Floquet-induced transitions. Systematic screening identifies 349 stacking configurations, derived from 80 monolayer layer groups, that satisfy the necessary group-subgroup symmetry conditions. First-principles calculations on bilayer ${\mathrm{NiCl}}_2$ with $D_{3d}$ and $C_{2h}$ symmetries confirm the emergence of light-induced spin splitting consistent with the predicted altermagnetic spin point groups. These findings establish a Floquet-based framework for designing dynamically tunable 2D altermagnets and point toward potential applications in spintronic devices and low-dimensional magnetic systems.

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• Magnetic phase transitions
• Spintronics
• Electronic structure
• Symmetries