The Spanish Football Federation (RFEF) disciplinary opted to hand the Dutchman the shortest permitted suspension after his sending off in the final minute for arguing with officials as 10-man Barcelona limped to a goalless draw.
Referee Carlos del Cerro Grande wrote in his report that Koeman was sent off for "protesting to the fourth official in an ostentatious manner, with his arms raised, shouting and leaving the technical area, complaining: 'hey, the referee has to whistle, the referee has to whistle, dammit, he has to whistle' even though he had already been warned by the fourth official to stop protesting at the referee's decisions".
Koeman wanted play stopped because there were two balls on the pitch.
"In this country you get sent off for nothing," Koeman told the press conference after the match, explaining that "it was for telling the fourth official that there was a second ball on the pitch, that he had to stop the game."
Koeman is under pressure because Barcelona are already seven points behind Liga leaders Real Madrid, although they have a game in hand, and have scored ust five goals in five league matches.
He will not be able to sit on the bench on Sunday as Barcelona host Levante and on October 2 when Barcelona visit champions Atletico Madrid.
The disciplinary committee also banned midfielder Frenkie de Jong for one match.
The Dutchman was sent off in 65th minute for his second yellow card in four minutes.