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 Soinning-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 unusual stories.
He added: "There are many sightings of a child ghost that walks on the bridge.
