Police were called to Russell Street in Lincoln at 1.56pm this afternoon after reports that a body had been seen in the river.
Officers attended the scene and the dead body of a man was found.
The area has been sealed off to preserve the scene and allow officers to carry out initial investigations.
At this stage the death is being treated as unexplained and work is being conducted to establish what has taken place.
Anyone with any information can call 101, or call independent charity Crimestoppers anonymously on 0800 555 111.