So how did it get there, and is this the weirdest thing to happen to a letter box?
BaffledThe incident became even more unusual when it emerged that Uri Geller lives in the village. He told the Telegraph that there were stories of the ghost of a child that was on the bridge, and suggested she might have installed it.
A spokesman for Royal Mail, told the BBC: "The recent appearance of a postbox frontage on the side of the river bridge at Sonning is a mystery to us."
It also confirmed that it was just the front of a square letterbox that had been attached to the bridge - so there wasn't any chance that letter could be posted there.
TheoriesThe Daily Mail suggested it may have been placed there as a stunt by the producers of a play that was in the village in July - it features the tale of a man who holds up a village post office.
Post box odditiesWhoever is behind the post box stunt, can now lay claim to one of the oddest post box tales of recent years. However, they have some competition. Here are five of our favourites:
1. In the excitement over the Olympics in 2012, letterboxes were painted gold to celebrate each gold-medal winner. However, while a box was painted in Cornwall to celebrate where sailing star Ben Ainslie grew up, a man in Lymington in Hampshire, where Ainslie lives now, felt a box should be painted in the town - so decided to paint it himself.
He was initially arrested for vandalism, but after a campaign and intervention by Ainslie, the Royal Mail eventually withdrew its complaint and said the box would remain gold forever.
2. In October, a Cheltenham post box was found lying in the road. The police confirmed there hadn't been any accidents in the road, and Royal Mail employees have commented that they are too heavy to be uprooted and pushed over. No-one knows how this postbox ended up on its side.
3. At the end of last year, builders renovating New Street Station took a post box off the wall. It was then that they discovered it was full of letters - some dating back to 1989. It emerged that somehow Royal Mail had overlooked the box, and it hadn't been emptied in 23 years.
4. Meanwhile, a post box in West Kensington was placed too close to a plane tree. The trunk has started growing over and around the box, which makes it look as though the box is being consumed by the tree. It can be seen here.
5. In 2009 a postbox in Edinburgh was wrapped in paper, complete with a red bow. The Royal Mail suggested it could have been a romantic gesture, and confirmed that the mail inside was unharmed.
Royal Mail shuts post box over parking fine
Royal Mail refuses to deliver to OAP with 'slippy path'
Royal Mail cuts lost post compensation