Koeman was dismissed after a 1-0 defeat by Rayo Vallecano left the team sitting ninth in La Liga.
Barca have won only two of their last seven league games under Koeman, a run that included losing to Real Madrid in Sunday's Clasico at Camp Nou.
A poor start to the season has raised the possibility of the team failing to qualify for next season's Champions League, which would have enormous consequences for the club's already dire financial situation.
Xavi, who is currently in charge of Qatari club Al Sadd, is considered Koeman's most likely successor.
Barcelona B coach Sergi Barjuan could take charge of the first team on an interim basis, with games against Alaves, Dynamo Kiev in the Champions League, and Celta Vigo to come before the next international break.
