Southampton have sacked manager Nigel Adkins after two and a half years in charge and appointed Argentine Mauricio Pochettino as manager.
The Saints and are currently three points clear of the relegation zone.
Adkins, 47, took the club from League One to the Premier League with successive promotions.
Pochettino, 40, will take charge against Everton on Monday.
The Argentine left Spanish side Espanyol in November and will lead training on Saturday for the first time.
Southampton chairman Nicola Cortese said in a statement: "This decision has been made with the long-term ambitions of Southampton Football Club in mind.
"Whilst we acknowledge the contribution Nigel has made during the past two years, for the club to progress and achieve our long-term targets a change was needed.
"Mauricio is a well-respected coach of substantial quality who has gained a reputation as an astute tactician and excellent man manager.
"I have every confidence that he will inspire our talented squad of players to perform at the highest possible level."
Pochettino played for Espanyol and Paris Saint-Germain as a defender during his career, and won 20 caps for Argentina, fouling Michael Owen at the 2002 World Cup to give England a penalty which was converted by David Beckham.
Pochettino said: "This is the kind of opportunity that any coach would relish. Southampton is a club with great heritage, and an even more exciting future.
"There is a clear vision to take the club to a new era of sustained success in the Premier League, and beyond, which I'm delighted to be part of."
Pochettino took charge of Espanyol at the age of 36 in January 2009, and managed a best finish of eighth in La Liga in 2010-11.
He left the club by mutual consent at the end of November 2012, with an overall record of 49 wins from 146 league matches.
Former Southampton manager and vice-president of the League Managers' Association Lawrie McMenemy believes Adkins has been treated harshly.
He told BBC Radio 5 live: "Well I am shocked. I said right from the start that to finish fourth bottom in the first season in the Premier League is success.
"The ambition this year is to stay in the Premier League and they look set for that, not many teams will go to Chelsea and get a draw.
"With due respect to Pochettino, what does he know about our game? What does he know about the Premier League?
"What does he know about the dressing room, does he speak English?
"Everybody will be feeling sorry for Nigel Adkins. I don't think he'll have a problem getting another appointment."
Chelsea boss Rafael Benitez was also surprised to see Adkins go just three days after Saints fought back for a point at Stamford Bridge.
He said: "'It is a surprise, he was doing a great job. He had problems at the start of the season but now he is doing well.
"Pochettino has been in Spain for years with Espanyol, I think he is a good manager but I am disappointed a manager has lost his job.''
Southampton had a tough start to life back in the top flight this season, but Adkins had steered the club out of the relegation places with a run of only two defeats in the last 12 league matches.
Adkins twice broke the club's transfer record last summer, first by signing for a reported fee of £7m, and then