The angry fans saw Koeman, surrounded the car and started knocking on it, shouting insults at the coach who was able to leave the scene without any physical damage - without leaving the car, which the fans demanded.
The incident is considered so serious that the club went out in a condemnation.
About a hundred fans had stayed
FC Barcelona condemns the violent and contemptuous actions our coach was given when he left Camp Nou.
The club will take security and disciplinary measures so that such incidents will not happen again ", the club writes on Twitter.
Around a hundred of the 86,400 who watched the match at Camp Nou had stayed behind to demand an explanation from the coach after the 1-2 loss.
Barca have started the season with just four victories in the first nine matches of La Liga.
The Dutchman has not commented on the incident himself, another of many setbacks this autumn.