A statement from Athletic said Marcelino, whose last job was with Valencia, had agreed to a contract until June 2022 and will be officially presented next week.
Garitano, who was sacked only hours after beating Elche 1-0 on Sunday, became Athletic’s coach in December 2018 after being promoted from the reserve team manager.
Marcelino’s first match is set to be Wednesday’s La Liga clash against Barcelona, while he will lead the team into the Spanish Super Cup later this month, meeting holders Real Madrid in the semi-finals.
He led Valencia to qualify for the Champions League in two successive seasons and won the 2019 Copa del Rey but was sacked the following September after falling out with owner Peter Lim.
Valencia have gone from bad to worse since Marcelino was dismissed. They are in La Liga’s relegation zone.