Southampton have sacked manager Russell Martin after a 5-0 thrashing by Tottenham on Sunday left them nine points off Premier League safety.
Martin, 38, guided the Saints to promotion last year in his first season in charge, beating Leeds United in the play-off final at Wembley.
But the loss against Spurs was Southampton's 13th in the league this campaign from 16 games.
Their only win this season was against Everton in November, while the St Mary's club have scored a league-low 11 goals.
"Going into the start of the season, we all knew the challenges that we would face this year as we readjusted to life in the top flight, competing in the best and most competitive league in the world," said a Southampton statement.
"However, the reality of our situation is clear. The board have supported Russell and his staff and been open and transparent regarding our expectations.
"We have all been on the same page in recognising the urgency of needing results to improve.
"We would like to take this opportunity to thank Russell and his staff for all the hard work and dedication they have given the club on and off the pitch over the last 18 months.
"Everyone connected with Southampton FC will always have fantastic memories of last season, especially the play-off final win in May."
Southampton Under-21s manager Simon Rusk will take interim charge of the senior side and his first game will be a Carabao Cup quarter-final at home to Premier League leaders Liverpool on Wednesday.