A man who was out walking his dog in wooded area of Colyers Avenue, Corby, was assaulted by a passer-by who punched him to the back of the head and threatened him with a piece of wood.
The incident happened at about 8.40am on Saturday (October 27).
The offender is described as a white man, in his early 20s, 5ft 11in and with a skinny build.
He was wore a multi-coloured hooded jacket, light grey skin tight trousers and light grey trainers.
Anyone with information can contact Northamptonshire Police on 101 or Crimestoppers anonymously on 0800 555111.