Since moving to Edinburgh from Worcester in September 2003, the Scotland internationalist has been a valued member of the squad and has recorded 114 appearances - 96 starts - and scored 20 tries.
But Southwell, set to turn 29 in two week's time, has decided to swap the Black and Red for the Blue and Pink of Stade Francais this summer, although he says he will always have fond memories of his time at Edinburgh.
Southwell said: "I've had an amazing time at Edinburgh - it has been the best six years of my life and I've made some really good friends.
"But the opportunity came up to play for Stade Francais and I felt it was the right challenge for me.
"I'm looking forward to the next chapter in my career and I would like to thank everyone at Edinburgh who has helped me over the years.
"I've really enjoyed working with Andy (Robinson) and Moff (assistant coach Rob Moffat) over the past two seasons; I believe they have really improved my game.
"Since my first day at the club, I've always enjoyed a great relationship with the fans and I would like to thank them for their support.
"My wife and I will always call Scotland home and it has been a tough decision."
Head coach Andy Robinson thanked Southwell for his contribution to the Edinburgh cause and wished him well for the future.
Robinson said: "Hugo has been a tremendous servant to Edinburgh over the past six years and his contribution towards moving the club forward has been appreciated.
"The move to Stade Francais is a great opportunity for him and the management team would like to wish him well.
"But Hugo's departure will now give Jim Thompson to chance to challenge with Chris Paterson for the full-back position."
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