Beware of the dog (also rendered as Beware of dog) is a warning sign posted at the entrance to a building or other private area indicating that a dangerous dog is within. Such signs may be placed to deter burglary even if there is no dog, or if the dog is not actually a competent guard dog.^[1][2]

Warning signs of this sort have been found in ancient Roman buildings such as the House of the Tragic Poet in Pompeii, which contains a mosaic with the caption cave canem (pronounced [ˈkäu̯eː ˈkänɛ̃ˑ]).^[3] The Roman work Satyricon, written by Petronius, includes a passage mentioning the phrase cave canem painted on a wall with large letters, in the chapter Dinner with Trimalchio.^[4]

Philippians 3:2 is translated as "beware of the dogs" or "beware of dogs" in the King James Bible and many other editions.^[5] For example:

Beware of dogs, beware of evil workers, beware of the concision.

This is often interpreted as a euphemism, bad people having been described as dogs in a number of previous biblical passages.^[6] Nonetheless, the yard signs are sometimes alluded to in reference to the passage.^[7][8] The use of such signs in the Roman world may have influenced the author of the passage,^[9] and conversely the passage may have influenced the wording of the more modern yard signs.^[10]

The examples and perspective in this section may not represent a worldwide view of the subject. (November 2023)

Under English law, placing such a sign does not relieve the owner of responsibility for any harm which may come to people attacked by the dog.^[11][12] Where a company employs the services of a guard dog, Section 1(3) of the Guard Dogs Act 1975 requires "a notice containing a warning that a guard dog is present is clearly exhibited at each entrance to the premises."^[13] In many cases, security signs integrate both CCTV warnings and Guard Dog warnings into the same signage.^[14]