Phase change materials (PCMs) have recently garnered significant attention for thermal energy storage applications. However, widely used PCMs, such as polyethylene glycol (PEG), suffer from low heat conductivity, poor thermal stability, and a tendency to leak, limiting their practical use. Therefore, the objective of this study is to develop a highly thermally conductive composite PCM using PEG and a thermally conductive nanomaterial, specifically cellulose nanocrystals supported by copper nanoparticles (CNC@Cu-NPs). The CNC@Cu-NPs nanocomposite material was used as a matrix to support polyethylene glycol (PEG), resulting in a shape-stable phase change material termed PEG/CNC@Cu-NPs. The obtained PEG/CNC@Cu-NPs were characterized using various analytical techniques. Fourier transform infrared spectroscopy (FTIR) analysis revealed the chemical linkages between PEG and cellulose nanocrystals through a radical polymerization reaction. Polarizing optical microscopy (POM) images of PEG/CNC@Cu-NPs exhibited spherocrystal morphology with smaller sizes compared to pure PEG, suggesting that CNC inhibited PEG side-chain mobility. According to differential scanning calorimetry, the suggested PEG/CNC@Cu-NPs demonstrated a temperature transition between 0.51 °C to 24.5 °C with enthalpies of fusion and crystallization measured at 92.19 and 98.54 J/g, respectively, while also exhibiting excellent cycling stability after 60 heating/cooling cycles. The addition of a modest amount of copper nanoparticles resulted in a remarkable improvement (>50 %) in the thermal conductivity of the PCM. Thermal imaging using an infrared camera showed that PEG/CNC@Cu-NPs exhibited a delay of 10 min in heating compared to pure CNC, confirming the good heat retention capacity of the developed composite material. Based on the aforementioned findings, the proposed PEG/CNC@Cu-NPs PCM offers promising application possibilities in the field of thermal energy storage.