The use of oriented macroscopic roughness to enhance the effective adhesion between two immiscible polymers was demonstrated. Bilayer specimens of polycarbonate and poly(styrene-co-acrylonitrile) (SAN) were produced with rough interfaces by scribing grooves of varying depths and spacings into the polycarbonate (PC) before joining the layers. The SAN, having a significantly lower glass transition temperature than polycarbonate, flows into the grooves during annealing at temperatures just over the PC glass transition, creating a mechanically interlocking interface. Subsequent measurements of bilayer interfacial fracture toughness showed up to a twenty-fold increase from that of a smooth interface when grooves were oriented perpendicular to the direction of interfacial crack propagation. The increase in toughness was shown to be greater as grooves were spaced closer together, and as groove depths were increased. Propagation of interface cracks followed a stick-slip mechanism, slowing considerably at each groove. Analysis of fracture surfaces indicates the increase in toughness to be mainly due to cohesive failure and deformation of the polymers at the grooves. Interface toughness was also enhanced with grooves scribed parallel to the propagation direction, but to a lesser degree than with perpendicular orientation.