The incident happened at Scarva Walk in Banbridge at about 16:00 BST on Friday.
The victim, who is aged in his 40s, was taken to hospital for treatment for head and facial injuries.
Two men in their 20s were arrested and are being questioned by police on suspicion of causing grievous bodily harm with intent.
One of the men is also suspected of assaulting police and causing criminal damage.
He was taken to hospital for treatment for a hand injury.
Det Sgt Colin Patterson of the Police Service of Northern Ireland said: "Anyone who saw this assault taking place and has information that could be useful to police should contact us."
Two people have been arrested after a man suffered head injuries in an serious attack in County Down.