A man who drunkenly punched a stranger at a Perth bar, knocking him unconscious and causing serious head injuries, has been jailed for a year.

Tristan Ceccato, 22, and Brent Robert Johnston, 21, were at the Amplifier Bar in January when they bumped into each other.

Johnston then punched Mr Ceccato in the back of the head and he fell face first on the floor, injuring his head, face, teeth and jaw, the WA District Court heard on Tuesday.

Judge John Staude described it as a "cowardly blow" against Mr Ceccato, who required surgery, and dental and physiotherapy treatment.