Functional load-bearing materials based on phase-change materials (PCMs) are under rapid development for thermal energy storage (TES) applications. Mesoporous structures are ideal carriers for PCMs and guarantee shape stability during the thermal cycle. In this study, we introduce transparent wood (TW) as a TES system. A shape-stabilized PCM based on polyethylene glycol is encapsulated into a delignified wood substrate, and the TW obtained is fully characterized, also in terms of nano- and mesoscale structures. Transparent wood for thermal energy storage (TW-TES) combines large latent heat (∼76 J g–1) with switchable optical transparency. During the heating process, optical transmittance increases by 6% and reaches 68% for 1.5 mm thick TW-TES. Characterization of the thermal energy regulation performance shows that the prepared TW-TES composite is superior to normal glass because of the combination of good heat-storage and thermal insulation properties. This makes TW-TES composites interesting candidates for applications in energy-saving buildings.