Abstract: This study investigated the microstructural evolution and mechanical behavior of Ti-4Al-0.005B alloy friction stir welded (FSW) T-joints and annealed at 600 °C and 700 °C, using multi-scale characterization to reveal relationships between annealing temperature, microstructure, and properties. Results show that annealed at 600 °C significantly promotes dynamic recrystallization (DRX) within the stir zone (SZ), resulting in a fine equiaxed α-phase microstructure, with a recrystallization fraction of 69.27% and a high-angle grain boundaries (HAGBs) content of 74.48%. In contrast, the thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) exhibit incomplete recrystallization, retaining a substantial fraction of low-angle grain boundaries (LAGBs). Annealed at 700 °C enhances DRX driving force but also causes abnormal grain growth, reducing the SZ recrystallization fraction to 52.25% and HAGBs content to 51.90%. Kernel Average Misorientation (KAM) analysis reveals a decrease in residual dislocation density at 700 °C, indicating enhanced recovery and corresponding grain coarsening, which contributes to a decrease in peak hardness (from 340.7 HV to 332.1 HV). Texture analysis shows a pronounced weakening of basal texture in the SZ at higher temperature (intensity drop from 30 to 15 mud), corroborating the increased DRX activity. Mechanical testing demonstrates that while annealed at 700 °C slightly reduces strength, it significantly enhances ductility (elongation from 6.18 ± 0.35% to 7.25 ± 0.83%) and impact toughness (from 28.13 ± 2.13 J/cm2 to 47.81 ± 4.77 J/cm2). An annealing temperature of around 650 °C is recommended to balance strength and toughness in Ti-4Al-0.005B alloy FSWed T-joints.