Video images capture the attack in the upstairs room of Burger King on Mosley Street in Manchester city centre.
They show the culprit lashing out at his victim several times as diners continue to munch away on a busy Saturday afternoon.
It is thought the victim bought food before rowing with his attacker.
He was stabbed six times in front of families and children and was taken to Manchester Royal Infirmary where he was later discharged.
The attack took place at 5pm on March 14.
Det Insp Mark Hudson, of Grey Mare Lane CID, is hoping the footage will help trace the attacker.
He said: "This was a particularly horrifying incident because it happened in a packed restaurant and I know many children and their families were shocked by what they witnessed.
"I would urge people to take a close look at that footage and, if you know who is responsible for this vicious attack, contact us."
The knifeman was white, 5ft 10 and in his mid to late 20s.
He had short dark receding hair, wore a light top, dark bottoms and dark trainers with a white emblem on the side.
Police believe he was with another man of a similar age and height who wore dark clothing.
