In semi-auxetic sandwich plates, the Poisson’s ratio varies from positive value to negative and vice versa in the thickness direction. The elastic modulus can also be variable throughout the thickness, as previously applied to other composite materials. Considering variable mechanical properties in a sandwich plate, the buckling load may significantly increase. For the first time, this study investigates how the buckling load of symmetric three-layer semi-auxetic sandwich plates (a core and two faces) is enhanced. The rectangular sandwich plates’ edges are simply supported or fully clamped and are subjected to in-plane (combined biaxial and shear) loads. Their buckling equation is developed and solved using classical plate theory and applying the generalized integral transform technique (GITT). As the geometry and material are symmetrical relative to the sandwich plate midplane, the buckling load (coefficient) is obtained through an eigenvalue problem. The GITT gives more accurate and quicker results than the finite element analysis developed in the commercial software ABAQUS. The results show that the minimum buckling coefficient depends only on Poisson’s ratio distribution pattern throughout sandwich plate thickness. Furthermore, if the absolute value of core Poisson’s ratio is less than the faces, the buckling coefficient increases and vice versa. As the variations can approximately coincide with the saddle surface of a hyperbolic paraboloid, a concise formula is innovatively proposed to predict the buckling coefficient quickly. The suggested formula represents satisfactory results when the core and faces Poisson’s ratios are between − 0.5 and 0.3.
