The victim was robbed of his phone in the grounds of Rushden and Higham Rugby Club on Bedford Road, at about 3pm on Saturday (June 25).
Later, at about 4.50pm, he was found unconscious at Kingsmead Park, also on the Bedford Road.
He had facial injuries which needed hospital treatment.
The offender is described as a 25-year-old, 6ft white man.
Officers would like to speak to anyone who saw the victim walking on the Bedford Road on Saturday afternoon or witnessed the robbery or the assault, or who has any information regarding these incidents.
Witnesses or anyone with information about this incident can contact police on 101 or Crimestoppers anonymously on 0800 555111.