The once-capped All Black admitted that a fresh challenge was the right move for his family but that the Scarlets will always be his club.
"I'd like to thank everyone at the Scarlets, the coaching team, the players and all the supporters for a great six years," said King.
"It's been a massive honour to be part of this great club, with its history and spirit. My family has always been made to feel really at home here and we've enjoyed our time in Llanelli - we've made great friends and our children were born here so it will always be a special place for us.
"You've got to admire the work being done by the Scarlets to develop great new rugby players from within this region and, from what I've seen, it will be a good thing for the Scarlets' future.
"It's been a tough decision to leave as I've got a real bond and affinity with the Scarlets and all the boys here as I've been here quite a while. They're a really good bunch of players and it's a great atmosphere in the group. It's been a pleasure to train and play alongside them and see some of the younger guys coming through so well now and many of them getting international caps.
"At this stage in my career, the offer from France was one that was right for me and my family and I felt that it's probably time to move on. I'm grateful to everyone associated with the Scarlets and I'll go with great memories of both Stradey Park and Parc y Scarlets."
King has been a key figure in Llanelli since joining from Stade Francais in May 2005.
The 30-year-old playmaker has made 135 appearances for the Scarlets and has recently played a key role in the development of up-and-coming midfield stars such as Jonathan Davies and Gareth Maule.
A fans' favourite, head coach Nigel Davies praised King's contribution after announcing his departure this afternoon.
"Regan's contract with us concluded this summer and he goes with our best wishes to France where at this stage in his career he'll have some great opportunities," said Scarlets head coach Nigel Davies.
"The French clubs are staking a major claim for some of the top players in Europe at this time and evidently have the budgets and resources to do so.
"Regan has been a great asset within our rugby set-up for many years now and as a player has skills and qualities which you just don't find everywhere. He's a true rugby talent.
"His contribution and experience as one of our senior players has been particularly significant in recent seasons - our backs have without doubt benefited from playing and training alongside him, appreciating the quality he brings to the game and learning from his great skills, timing and ability with the ball.
"We're very grateful to Regan for his contribution over the past six seasons and wish him and his family well in France."
With a number of young stars making a name for themselves so far this term, Davies suggested the Scarlets will be making a series of announcements on next season's playing squad once the Six Nations comes to an end in March.
The Scarlets are currently sitting second in the Magners League table having produced some truly outstanding rugby this season and Davies is understandably confident that his side are moving in the right direction.
"I am very comfortable with what we have achieved in terms of securing the talents of our core Scarlets players going forward," added Davies.
"This group has progressed significantly over the past 18 months and we will continue our investment in developing Welsh talent from within our region - that is key to our philosophy and values and our future direction as a rugby business.
"We expect to make these announcements post Six Nations when our whole group is back together within our environment here at Parc y Scarlets."