Celta have won just one game this season and are one point and one place outside the relegation zone.
Garcia was appointed as Celta manager in November last year, replacing Fran Escriba in the role, with the club in the relegation zone.
The 46-year-old helped the Celticos avoid relegation on the final day of the season and was handed a new two-year contract in the summer.
But another false start to the campaign has seen the club make Garcia the first managerial casualty of the season.
The former Barcelona midfielder’s final game in charge of Celta came in Friday’s 1-1 draw at Huesca.
"Oscar Garcia Junyent and his coaching staff will no longer coach RC Celta," a club statement read.
"The club conveys its sincere gratitude, both to the Catalan coach and to his team, for the work, professionalism and effort they've shown throughout their time at RC Celta, in which they managed to consolidate LaLiga status during a complicated 2019-20 season.
"RC Celta wishes Oscar Garcia and his assistants the best of success in the future, both professionally and personally."
Garcia has previously spent time managing Maccabi Tel Aviv in Israel, Brighton and Hove Albion, Watford, Saint Etienne, Olympiacos and RB Salzburg.
Celta’s next game is at Sevilla after the international break on November 21.
