The Bright Red Letter Box frontage appeared about a metre above water level on a parapet of the bridge, which crosses the river.
Villagers of Sonning-on-Thames, including Uri Geller, have been left puzzled as to how people could post letters or collect them from the box which is impossible to reach on foot and liable to flooding.
Mr Geller, who has lived in the Berkshire village for three decades, said: “I have never seen anything like this anywhere in the world, it’s a new one on me.”
But Mr Geller said the village had been known to have its rather unusual stories.
He added: “There have been many sightings of a child ghost that walks on the bridge.
“Maybe it was the ghost of a mischievous little girl.”
As well as being difficult to reach it is also thought the letter box could be impossible to use as the area of the Thames it occupies often floods.
The traditional letter box frontage, which is around 100 years old, was spotted by a Royal Mail delivery man who took a photo to prove where the letter box had been positioned and that he would need a boat to collect any post.
A spokesman for Royal Mail, said: “The recent appearance of a postbox frontage on the side of the river bridge at Sonning is a mystery to us.
“It is certainly not an operational posting facility and we have no knowledge of how it arrived at this specific location.”