A knocker is a metal device attached to the front of a door that visitors use to announce their arrival by banging it against the door. Before doorbells became common, knockers were the standard way to let someone know you were outside. They usually consist of a hinged metal piece (often shaped like a ring, a lion's head, or another decorative design) that you lift and let fall against a metal plate, creating a loud knock-knock sound that echoes through the house.

Knockers have been used for centuries, and many old homes still have them today. Some are simple and practical, while others are ornate works of art that show off the homeowner's style or wealth. Victorian houses often featured elaborate brass knockers shaped like animals or mythological figures.

The word can also mean someone who knocks: a door knocker is someone who goes door to door, perhaps selling something or campaigning for a political candidate. In British slang, a knocker can also be someone who criticizes or finds fault with things, though this usage is less common.