The victim, who works at the Es Baluard restaurant in the Puig de Sant Pere where the King of Spain once entertained actor Antonio Banderas and his wife Melanie Griffiths, was stabbed in the chest as he made his way from the office to the bathroom on the second floor.
Just as he walked out of the office, he was attacked by the man who had apparently grabbed the knife from a drawer downstairs.
Neighbours were quick to rush to the victim's aid.
They found him conscious but losing a lot of blood - he had been seriously wounded in the attack.
While the alarm was raised and the police were called, neighbours also discovered that some documents had been set alight in the waste paper bin in the restaurant's office.
The close-knit neighbourhood was shocked by yesterday's violent attack and members of the National Police and forensic teams spent all of the day at the scene trying to figure out how the attacker gained access to the second floor when the restaurant was still closed. They were also hunting for the weapon.