To obtain high-entropy carbide (HEC) joints with excellent high-temperature performance, a (HfZrTiTaNb)C HEC joint featuring a direct diffusion-bonded interface with a Nb-based interlayer was successfully fabricated at relatively low temperatures of 1150–1250 °C for 60 min under 10 MPa. Starting from a modified Ni/Nb/Ni composite interlayer with a Nb content of > 64 at%, an alloyed Nb₂Ni layer was constructed in situ by accelerating the directional diffusion of Ni atoms from the high-entropy interface into the remaining pure Nb through the Ni‒Nb eutectic liquid. Moreover, the excess liquid phase was squeezed out of the bonding region, ensuring the absence of Ni-based compounds. Leveraging the intrinsic interfacial stability and sluggish diffusion effect, the HEC with its original lattice structure, was capable of developing diffusion bonding with the Nb₂Ni layer instead of interacting with the liquid phase. The high reliability of the HEC/Nb₂Ni bonded interface was confirmed by the coherence of (1 $\overline{\text{1}}$3) _HEC// $(141{)}_{{\mathrm{N}\mathrm{b}}_2\mathrm{N}\mathrm{i}}$ and a calculated lattice misfit of 0.044. The HEC joint had a high room-temperature strength of 174 MPa because of the homogenous Nb₂Ni layer, which exhibited nanohardness (15.2±1.5 GPa) and an elastic modulus of 219.9±17.5 GPa. Furthermore, the strength of the HEC joint did not decrease at 1000 °C, increasing by ~49% over that of HEC/Ni/HEC diffusion-bonded joints, which have stringent surface flatness requirements. This suggested that the HEC/Nb₂Ni interface had excellent resistance to high-temperature softening, even though it was invariably the initial failure location. This work is informative for designing bonding structures and preparing HEC components.