In this study, SiC and Ga2O3 substrates were bonded under atmospheric conditions using an extremely thin amorphous layer (∼7 Å). Conventional wafer-bonding techniques employ an oxidizing treatment for surface functionalization, resulting in the formation of an oxide layer at the bonding interface. This study demonstrates the bonding of a hydrofluoric-acid-treated SiC surface with a plasma-activated Ga2O3 substrate, where the -OH groups on the SiC and Ga2O3 surfaces form direct bonding via a dehydration reaction at 250 °C. The interfacial analysis indicates that bonding using the reduction treatment instead of oxidization reduces the thickness of the intermediate layer at the SiC/Ga2O3 interface, which is a thermal and electrical barrier. In addition, it is remarkable that the SiC and Ga2O3 substrates are directly bonded by generally used surface cleaning processes, contacting the surfaces under atmospheric conditions, and annealing at 250 °C. We believe that the bonding process using the reduction process can contribute to future heterogeneous devices based on integrated dissimilar substrates.