This study modeled and analyzed the thermomechanical buckling behavior of smart magneto-electro-elastic (MEE) sandwich nanoplates using nonlocal elasticity, strain gradient elasticity, and higher-order plate theory. The sandwich nanoplate consists of ceramic and metal functional graded foam structure in the core layer and is composed of magneto-strictive and electro-elastic materials in the surface layers. Due to the functionally graded feature in the core layer, pure metal/metal foam, pure ceramic/ceramic foam, and metal + ceramic foam structures are modeled. The foam structure can be distributed uniformly and symmetrically throughout the thickness of the core layer. The effects of nonlocal elasticity, strain gradient elasticity, foam distribution, and foam void ratio of the core layer on the thermomechanical buckling behavior of the smart sandwich nanoplate have been examined in a broad framework. Additionally, the effects of electro-elastic and magneto-strictive material characteristics of smart surface plates on thermomechanical buckling response were examined according to the applied external electric and magnetic potential intensities. It is observed that the foam structure and foam void fraction ratio in the core layer are effective on the thermomechanical buckling behavior of the smart sandwich nanoplate. Moreover, it is concluded that the applied external electric and magnetic potential can change the thermomechanical buckling behavior of the sandwich nanoplate.