Torrefaction is a key technology for upgrading biomass, which typically has high moisture content and low calorific value, into a high-quality solid biofuel. However, its effectiveness is highly dependent on the specific feedstock and operating parameters. This study investigates the torrefaction of hazelnut shell, an abundant agricultural residue in Türkiye, to examine the effects of process temperature and retention time on the torrefaction process and to determine better process conditions. Lab-scale experiments were conducted at temperatures of 260 °C, 280 °C, and 300 °C and retention times of 30 and 60 min. At the most severe condition of 300 °C for 60 min, the mass yield decreased to 59.44%, while 78.38% of the feedstock’s original energy was successfully retained in the final torrefied biomass. The Energy Densification Ratio (EDR), another significant indicator for quantifying energy concentration, consistently increased with process severity as low-energy volatile compounds were removed. A maximum EDR of 1.32 was achieved at 300 °C and 60 min. These results demonstrate that torrefaction can convert hazelnut shells into a carbon-rich, energy-dense biofuel suitable for high-energy applications.