The Premier League club, fifteenth in the table after 22 games, ousted exportero Nigel Adkins, 47, as coach after winning just two of their last ten games.
Pochettino, 40, will take charge of his first training in Southampton on Saturday, two days before it will be their first meeting in front of his new team on Monday against Everton.
On 26 November, the Espanyol terminated the contract of Argentine exdefensa after almost four years as head of whitish bench when the team remained as bottom team in the Spanish League. Pochettino had come to set Barcelona on January 20, 2009, after the dismissal of coach Jose Manuel Esnal 'Mane', as an alternative to try to save the club from relegation.