Police are searching for a man who assaulted an elderly man and his dog at a western Sydney park last night.

A 72-year-old man was treated for facial injuries after being punched several times while walking his dog in a reserve on Grantham Road at Seven Hills.

Police spokesman Senior Constable Chris Nash says the attacker was provoked when the man's dog barked at him.

"That man actually approached the 72-year-old and his dog, he took the leash from him and swung the dog around," he said.

"He has then actually punched the elderly man, who fell to the ground, and continued to punch him and kick him."