Piezoelectric energy harvesters convert ambient vibrations into electrical power, yet conventional cantilevers mainly exploit the longitudinal d31 mode. This paper studies how the effect of the substrate’s effective Poisson’s ratio (ν) can be isolated and tuned to generate significant power with the transverse d32 mode. Honeycomb (ν > 0), Neutral (ν ≈ 0), and Auxetic (ν < 0) unit cell structures are compared in terms of power output. A novel Strain-Enhanced Metamaterial Cantilever (SEMAC) is proposed to increase the effect of these structures on the power output. A multiphysics finite element model and experiments show that a negative Poisson’s ratio leads to an amplified stress in the piezo’s y-direction, generating significant power from the d32 mode. Under equal tip displacement, the Auxetic harvester delivers +90% power relative to the Neutral, whereas the Honeycomb yields −64%; at equal 0.2 g rms base acceleration, the increase is +77% for the Auxetic and −72% for the Honeycomb relative to the Neutral. The approach isolates the substrate’s effective Poisson’s ratio as the driver of the change in the power output.