The 52-year-old victim had been punched outside the Three Hulats in Chapel Allerton by a man who left the scene before police arrived.
A West Yorkshire Police spokesman said they had been called to the pub in Harrogate Road just after 7.30pm.
He said: "Officers attended and the 52-year-old victim was taken to hospital by ambulance for treatment to a head injury. His injuries were not considered life threatening.
"A scene was put in place for forensic examination and was removed shortly before midnight."
Anyone who witnessed the assault or who has any information is asked to contact officers via 101, quoting crime reference 13180406042.
Alternatively, call the independent charity Crimestoppers anonymously on 0800 555 111.