Abstract Ultra‐high temperature ceramics (UHTCs) are a group of advanced ceramic materials that possess excellent high temperature capabilities, which make them especially suitable for extreme environment engineering applications. As an effective assembling method, joining is frequently required for fabricating sophisticated structures for such applications due to the excessive challenges and costs in producing near‐net shapes. Here, we introduce a promising new joining technique to effectively join UHTCs called Instantaneous Nanowelding , which uses direct electric current assisted rapid Joule heating to instantaneously bond hafnium diboride (HfB 2 ) to zirconium diboride (ZrB 2 ) in 1 s down to atomic scale. Our approach is analogous to high temperature spot welding, and the entire process is complete in 10 min, and the instant diffusion occurs in 1 s. Seamless HfB 2 /ZrB 2 interfaces are formed at 1750 for a duration of 1 s. A series of characterizations are done at the interfaces using techniques including SEM, WDS, EBSD, and S/TEM to observe Zr x Hf 1− x B 2 solid solution formation. Highly coherent transition with perfect lattice alignment at atomic scale from ZrB 2 to HfB 2 is observed using S/TEM, meaning that the two materials are brought to atomic contact.