<p>Mechanical interfaces—in which two materials are simply pressed together—are common in electronic systems, but microscopic surface roughness limits the area that is actually in contact, creating an impedance to electron or phonon transport. This can lead to Joule heating in electronic circuits and poor heat dissipation in power electronics. Here we show that mechanical metainterfaces, which are inspired by traditional carpentry joints such as mortise and tenon and finger joint, can enhance interfacial electronic and thermal transport. The interfaces use geometry-driven contact force augmentation in combination with conversion of interfacial stress from compressive to shear, which removes surface dielectric barriers. As an example for electrical interfaces, we create a mortise–tenon joint used in a plug-in connector for electric vehicles that exhibits one-eighth the area-normalized electrical resistance of its commercial counterparts. For thermal interfaces, we create a finger-joint interface used in electronic cooling that exhibits a thermal resistance as low as 2.3 K mm² W⁻¹, resulting in an additional chip temperature drop of 44 °C compared with current solutions when used between a light-emitting diode chip and a copper heat sink. The metainterfaces can be made with regular machining as the patterning technique and are readily scalable.</p>