This study presents a numerical investigation of a piezoelectric cantilever beam incorporating a reentrant elastic substrate to enhance energy harvesting performance. The system consists of an elastic substrate bonded with piezoelectric patches mounted on the surface. A reentrant lattice geometry, known for showing a negative Poisson’s ratio, is used as the substrate. The effective mechanical properties of the reentrant structure are determined using a three-dimensional representative volume element (RVE) with periodic boundary conditions, following established homogenization approaches for cellular materials. The homogenized effective properties are used to model an equivalent homogeneous reentrant substrate and are compared with simulations performed using the actual reentrant geometry. Cantilever beam simulations are carried out by fixing one end and applying a point load at the free end, and the resulting electrical output voltage is evaluated. The results demonstrate that the output voltage obtained using the effective properties of the reentrant substrate closely matches that predicted using the detailed reentrant geometry, yielding approximately the same electromechanical response. In addition, the reentrant substrate generates a significantly higher output voltage compared to a conventional solid substrate because of enhanced strain amplification. These findings validate the homogenization strategy and highlight the effectiveness of reentrant substrates in improving piezoelectric energy harvesting performance.