<p>To overcome oxide-film barriers, low bonding efficiency, and interfacial defects during diffusion bonding of high-strength 6061 aluminum alloy, an Al-Zn-Ge ternary interlayer was designed for low-temperature SPS-assisted bonding. In this study, Al-xZn-yGe interlayers with different Zn and Ge contents were prepared, and their effects on the microstructural evolution, phase constitution, thermal behavior, deformation behavior, and mechanical properties of 6061-T6 aluminum alloy joints were systematically investigated. The results show that Ge addition promotes the formation of a dense Ge-containing eutectic network and reduces the bonding temperature from 530 to 460 °C. Zn acts as an effective modifying element by regulating the distribution of Ge-rich regions and promoting the formation of a refined α + η + β ternary eutectic structure, thereby improving the microstructural uniformity and bonding quality of the interlayer. Owing to reduced thermal exposure, enhanced interfacial diffusion, improved bonding density, and possible Mg-Ge-related interfacial reactions, the joint bonded with the Al-20Zn-15Ge interlayer exhibits the best overall performance, with a maximum tensile strength of 154.42 MPa and an elongation of 2.23%. However, excessive Zn addition at 30 wt.% causes liquid-phase overflow and defect formation, resulting in deteriorated joint integrity. The bonding process is more appropriately described as a diffusion-dominated transient liquid-phase–assisted bonding process rather than conventional liquid-state brazing. This study clarifies the compositional regulation mechanism of Zn and Ge in Al-Zn-Ge interlayers and provides guidance for designing high-performance interlayers for low-temperature aluminum alloy bonding.</p>