Thomas Matthews landed the blow outside the Phoenix pub in Oldbury on 24 October last year.
The victim, aged in his 40s, fell to the ground after being punched in the face and suffered serious head injuries.
Matthews fled following the attack in Martley Road but he was identified from CCTV and arrested within days.
The 29-year-old told officers he’d initially tried to calm down the partner of the victim after she’d been involved in an earlier altercation in the pub and was making threats.
The victim was with her by the smoking shelter and Matthews claimed he appeared to lunge towards him. Believing he may have had a weapon he struck back.
However, our investigations proved there was no weapon and no attempt by the victim to attack Matthews.
It led to Matthews, of Bredon Court, Halesowen, admitting wounding and he was jailed for three years and six months at Wolverhampton Crown Court on Friday (19 February).
DC Tamara Collis, from force CID, said: “The victim has been left with serious brain injuries which will forever impact on his life.
“This case sadly highlights that even just a momentary loss of control can have traumatic and life-changing consequences.
“Only Matthews will truly know what his intentions were when he went across to where the victim was. But his subsequent actions have cost him his freedom and left a man with an entirely different life ahead of him.”