North Yorkshire Police says the hooded attacker struck as the victim was walking his dog in Hull Road Park, Alcuin Avenue, between 10.15pm and 10.45pm on Wednesday, August 14.
It happened near to the bridge and tennis courts, the force said.
A police spokesperson said: "The victim, who is aged in his 30s, managed to defend himself and did not suffer from any physical injuries, but was left very shaken.
"We’re particularly appealing for information about the suspect who is described as a white man, around 5ft 6in to 5ft 8in tall, aged late teens to early 20s, stocky build, no facial hair, smooth complexion and brown eyes."
The suspect was wearing a black hoodie with the hood up, the hoodie had three white stripes along the arms in an Adidas style, black jogging bottoms which matched the top and also had three white stripes down the side and dark trainers.
Please email lyn.knight@northyorkshire.police.uk if you have any information that could help the investigation.
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