Two men were standing in the lane near to Asda on Gorstan Road, in the Summerston area of Glasgow, at around 9.10am today when another man approached them and began speaking to them.
He then attacked one of the men, punching him and hitting him in the face with a hammer before running off towards Arrochar Street.
The 35-year-old victim was taken to the Western Infirmary and treated for his injuries before he was later released.
The suspect is white, around 5ft 9in with a stocky build. He was clean shaven and wearing a black hooded top with yellow stripes and a yellow logo on the front.
Police have been checking CCTV footage from the area and are appealing for witnesses.
Detective Sergeant Lesley Ann McGee said: "The victim has had to have treatment for a facial injury after this man lashed out for no apparent reason.
"We will be checking CCTV images from the area and the surrounding areas in an effort to find out more detail about the suspect. This is a busy area and there would have been a lot of people around at the time of the crime, although the suspect was wearing a dark-coloured top, it did have bright yellow stripes and a yellow logo which people may remember seeing.
"I am appealing to anyone who was in the area who may have seen the suspect or who may have any knowledge regarding this incident to get in touch with us."