Long processing time and high temperatures are often required in sintering ceramic electrolytes, which lead to volatile element loss and high cost. Here, an ultrafast sintering method of microwave‐induced carbothermal shock to fabricate various ceramic electrolytes in seconds is reported. Furthermore, it is also possible to integrate the electrode and electrolyte in one step by simultaneous co‐sintering. Based on this ultrafast co‐sintering technique, an all‐solid‐state lithium‐metal battery with a high areal capacity is successfully achieved, realizing a promising electrochemical performance at room temperature. This method can extend to other various ceramic multilayer‐based solid devices.