Thermal modification of wood is a sustainable treatment process that enhances its durability, dimensional stability, and resistance to biological degradation. This study investigates the chemical transformations of Daniellia oliveri wood, a lesser-utilized species, subjected to thermal modification at 160 °C, 180 °C, and 200 °C. Utilizing standards set by the Technical Association of Pulp and Paper Industry (TAPPI), the chemical composition of sapwood and heartwood was analyzed. Results reveal that hemicellulose content declined significantly with increasing temperature, with reductions of up to 34.94% in sapwood and 30.77% in heartwood. Lignin content increased proportionally, reflecting the thermal degradation of hemicellulose and cellulose and the relative enrichment of lignin. Similarly, extractive content, measured using acetone and ethanol, increased markedly, with the highest values observed at 200 °C, indicating enhanced migration and formation of extractable compounds. Cellulose content demonstrated stability, with modest increases, while holocellulose content showed a progressive decline due to hemicellulose degradation. These findings underscore the potential of thermal modification to enhance the performance of Daniellia oliveri wood, particularly its resistance to biodegradation and moisture. By improving its chemical properties, thermal treatment positions this species as a viable alternative to more durable yet overexploited tropical hardwoods, supporting sustainable forestry and expanding its market potential. The study concludes that thermal modification offers an environmentally friendly and economically advantageous approach to enhancing the utility of lesser-utilized wood species for high-value applications.