The workhorse Ti-6Al-4V alloy has numerous applications under extreme conditions, and there is a necessity to produce metallic joints with the alloy for fabrication of some industrial components. In the present study, the solid-state diffusion bonding (DB) process was used to produce similar metal joints between Ti-6Al-4V alloy sheets with and without copper and nickel interlayers. The effect of the bonding temperature as well as the influence of interlayers on the joint interface microstructure and, subsequently, on the mechanical performance of the bonded joints were investigated. The DB was performed at three different bonding temperatures of 1,023, 1,123, and 1,223 K under high-vacuum conditions using a thermomechanical deformation simulator. The microstructure across interface of the joint was examined using optical and scanning electron microscopes. The joint strength was evaluated using both the nanoindentation technique and the shear test. The findings show the formation of intermetallic phases between titanium and interlayer(s) elements across the interface up to the bonding temperature of 1,123 K. Further increase in the bonding temperature led to a complete diffusion of the interlayers into the base metals, thereby forming the Widmanstätten microstructure across the interface. In the case of direct DB, there was a gradual closure of voids along with a homogenized joint interface with an increase in the bonding temperature. The variation of hardness across the bonding interface and the shear properties were found to be consistent with the resultant microstructural features evolved with the bonding temperature and type of interlayer(s).