Glynn Snodin has been appointed West Ham's first-team coach.
Snodin, 47, worked with Hammers boss Alan Curbishley at Charlton before joining Southampton as their first-team coach just over a year ago.
Saints boss George Burley said: "It's very disappointing to lose Glynn who is a smashing lad. He was very popular and will be a big miss for us.
"But he has the opportunity to go to the Premiership and feels this is an opportunity he could not turn down."
Burley added: "He has done well for us but this is a chance for him to work with a first-team in the Premiership and he felt the offer was right for him and his family."
West Ham have agreed a compensation package for Snodin, who recently greed to take a part-time role to coach the Northern Ireland side alongside Nigel Worthington.