A knocker (knacker) was a British spirit of the underground, which is thought to have originated in Wales and branch out to other areas such as Cornwall.

According to Cornish folklore the knocker was the helpful spirit of a previous fatality in the tin mines. A knocker is about 2 feet high, has a disproportionately large head, long beard and weathered, wrinkled skin. Their long arms almost touch the ground and they imitate the miner's clothes in dress, and carry such things as pickaxes and lamps.

Miners knew the knockers well but opinions on their intentions differed. Some believed the knocking on the cave walls caused by knockers were attempts to break down the walls, others that the knocks were directions on where to dig and others still that the knocks were warnings of an impending collapse. Knockers were said to be mischievious, at their best they hid tools and took candles, at their worst they set the tunnels on fire.

Other well known variants of knockers are the tommy-knockers, brought over to America by Cornish miners, the Welsh Coblynau and the Wichtlein.