Lyons, 30 joined the Scarlets in July 2008 on a four-year deal from the Waratahs. He leaves Llanelli with a personal message of thanks to the region's supporters and his fellow players and coaches.
He said: "It's been one of the most enjoyable times of my rugby playing career at the Scarlets and I have great respect for the commitment being made to develop new young talent at Parc y Scarlets. Credit should be given to the club for it's vision - there's some really good young players coming through now for the region and I'm pleased to have played my part in helping bring some of them through.
"Being part of the Scarlets has been something I've valued and feel honoured to have experienced - it's a really good young side and the guys play an exciting, attacking brand of rugby which has been great to be part of.
"This Scarlets team will only go from strength-to-strength with the core of players they have gaining in experience. You've got the quality and ambition of some good young blokes in the back row, they will put their body on the line and won't shirk anything."
Lyons, who appeared in 16 of the Scarlets 32 competitive fixtures this season added: "It's not an easy decision to move on, we've had a great few years in Wales and have been made to feel very much part of the Scarlets way of life in West Wales.
"However, I had always planned to play in France and I've been working closely with the region for some months now on my release. I'm looking forward to the opportunities ahead of me now and the chance to pursue my ambitions with a French club."
Scarlets Head Coach Nigel Davies said: "David has played an important role within our group over the past three years and he leaves knowing that he has made a significant impact on the squad of younger players we have coming through."
Davies added that that the back row was an area that the Scarlets were showing increasing strength in depth with the likes of Welsh internationals Josh Turnbull, Rob McCusker as well as emerging talent Ben Morgan.