A letter bomb, also called parcel bomb, mail bomb or post bomb, is an explosive device sent via the postal service, and designed with the intention to injure or kill the recipient when opened. They have been used in terrorist attacks such as those of the Unabomber. Some countries have agencies whose duties include the interdiction of letter bombs and the investigation of letter bombings. The letter bomb may have been in use for nearly as long as the common postal service has been in existence, as far back as 1764 (see Examples).

Letter bombs are usually designed to explode immediately on opening, with the intention of seriously injuring or killing the recipient (who may or may not be the person to whom the bomb was addressed). A related threat is mail containing unidentified powders or chemicals, as in the 2001 anthrax attacks.

Letter-bombs, along with anti-personnel mines, are typical examples of subject-matter excluded from patentability under the European Patent Convention, because the publication or exploitation of such inventions are contrary to the "ordre public" and/or morality (Article 53(a) EPC).