Wood is an eco-friendly and abundant substrate and a candidate for functionalization by large-scale nanotechnologies. Infiltration of nanoparticles into wood, however, is hampered by the hierarchically structured and interconnected fibers in wood. In this work, delignified wood is impregnated with gold and silver salts, which are reduced in situ to plasmonic nanoparticles via microwave-assisted synthesis. Transparent biocomposites are produced from nanoparticle-containing wood in the form of load-bearing materials with structural color. The coloration stems from nanoparticle surface plasmons, which require low size dispersity and particle separation. Delignified wood functions as a green reducing agent and a reinforcing scaffold to which the nanoparticles attach, predesigning their distribution on the surface of fibrous “tubes”. The nanoscale structure is investigated using scanning transmission electron microscopy (STEM), energy-dispersive spectroscopy (EDS), and Raman microscopy to determine particle size, particle distribution, and structure–property relationships. Optical properties, including response to polarized light, are of particular interest.