In this paper, a 2.5-D approach has been used to design 2.5-D miniaturized multifunctional active frequency-selective surfaces (2.5-D MMAFSSs). The unitary element, based on the via-connected metal strips and metallic parallel plates, can be miniaturized to 0.063λ ₀ , where λ ₀ is the free-space wavelength. To achieve the relative polarization symmetry, four similar subcells based on the unitary element are combined together to constitute a 2.5-D MMAFSS unit cell. The novelty of the proposed design lies in four independent operating states that can be realized by actively changing the on-off states of p-i-n diodes. In addition, the miniaturized configuration is beneficial for the easy realization of multiple electromagnetic states with great angle stability and polarization insensitivity. For a better understanding of the physical principle, the analyses of both surface current and electric field distributions are conducted at resonance frequencies. Also, the equivalent circuit models are established to further clarify the working mechanism of the structure. Finally, a prototype of the 2.5-D MMAFSS operating at 1.9 GHz is fabricated and measured to validate the proposed concept, and the experimental results are in satisfactory agreement with the simulated ones.