A 36-year-old man was punched in the face and robbed of a quantity of cash in Coronation Park near Corby town centre.
The incident happened at about 9pm on Sunday.
A police spokesman said the attack happened after the man left McDonalds in Corporation Street.
Two men approached him and they were joined by three other men.
All six men walked to the park where the robbery took place.
Three men have been arrested in connection with the incident and released on police bail pending further enquiries.