Summerfield will be working as first team coach with Steve Wigley. In an announcement to the club's official site, he said:
"I am delighted to be joining Southampton. It is a big opportunity and a great challenge, and I cannot wait to get started."
Coach John Blackley's future is as yet undecided, but a move to St. Mary's is still looking likely. Bobby Williamson is currently on holiday and so unavailable for comment, but it is expected he will now bring in backroom staff from his old club Hibs, having met up with coach Jim Clark last month.