We introduce a facile and effective fabrication of complex multimetallic nanostructures through block copolymer self-assembly. Two- and three-dimensional complex nanostructures, such as nanomesh, double-layered nanomeshes, and surface parallel cylinders on nanomesh, can be fabricated using the self-assembly of perforated lamellar morphology in block copolymer thin films. Simultaneous integration of various metallic elements, including Pt, Au, and Co, into the self-assembled morphologies generates multimetal complex nanostructures with highly interconnected morphology and a large surface. The resultant metal nanostructures with a large surface area, robust electrical connectivity, and well-defined alloy composition demonstrate a high-performance electrochemical catalysis for hydrogen evolution reaction (current density: 6.27 mA/cm(2)@0.1 V and Tafel slope: 43 mV/dec).