Bilbao fell to a 3-0 defeat at Levante on Monday, continuing a dreadful spell that has seen them fail to win a league game since the opening day of the season.
The only La Liga side besides giants Real Madrid and Barcelona never to be relegated from Spain's top flight, Athletic are now three points from safety with 11 points from 14 games.
"Athletic Club has decided to end the contract with coach Eduardo Berizzo," the club tweeted.
"From Wednesday December 5 Gaizka Garitano will be in charge of the first team."
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The 49-year-old Berizzo was diagnosed with prostate cancer in November 2017 while coaching Sevilla.
The coach overcame his battle with the disease but was sacked in December because of a bad run of form, taking over at Athletic in May 2018.
His replacement Garitano took Eibar to top-flight promotion for the first time in their history in 2014, although he resigned after they were relegated in 2015, despite the club being reinstated following Elche's demotion.
Garitano was sacked by Valladolid in 2015 and Deportivo La Coruna in 2017 before taking over at Bilbao Athletic, the club's reserve side.
(Reporting by Rik Sharma; Editing by Ian Chadband)
