A 47-year-old man needed hospital treatment after an unprovoked attack while walking his dog on Monday.
Police are appealing for witnesses to the assault in Dorset Gardens, Brighton.
The victim was walking his dog in Peace Park when he was assaulted at 10.30pm. He suffered a cut to his head.
The suspect is a white man, in his 30s, 6ft and of slim build, with brown hair in a ponytail.
He was wearing a dark-coloured hooded top with dark jeans and trainers.
Witnesses are asked to email firstname.lastname@example.org, call 101 quoting serial 1599 of 11/08 or call Crimestoppers anonymously on 0800 555 111.