An electronic thermometer includes an external casing unit, a circuit substrate, an electronic switch, a rotating arm unit, a probe unit and a waterproof unit. The external casing unit includes a top cover casing and a bottom cover casing, a receiving space is formed between the top and the bottom cover casings, and the bottom cover casing has a through hole communicated with the receiving space. The waterproof unit includes a first waterproof element and a second waterproof element. The first waterproof element is disposed between the top cover casing and the bottom cover casing and around the receiving space for preventing water vapor from entering the receiving space through the external casing unit, and the second waterproof element is disposed inside the through hole of the bottom cover casing for preventing water vapor from entering the receiving space through the through hole.