Ceramics possess high temperature resistance, extreme hardness, high chemical inertness and a lower density compared to metals, but there is currently no technology that can produce satisfactory joints in ceramic parts and preserve the excellent properties of the material. The lack of suitable joining techniques for ceramics is thus a major road block for their wider applications. Herein we report a technology to weld ceramic nanowires, with the mechanical strength of the weld stronger than that of the pristine nanowires. Using an advanced aberration-corrected environmental transmission electron microscope (ETEM) under a CO2 environment, we achieved ceramic nanowelding through the chemical reaction MgO + CO2 → MgCO3 by using porous MgO as the solder. We conducted not only nanowelding on MgO, CuO, and V2O5 nanowires and successfully tested them in tension, but also macroscopic welding on a ceramic material such as SiO2, indicating the application potential of this technology in bottom-up ceramic tools and devices.