The 27-year-old has played 43 Tests for Australia and represented the Wallabies at two World Cups.
"I met with David and his representatives in Sydney last week in what was an extremely productive meeting," said a delighted Scarlets CEO Staurt Gallacher.
"David outlined his reasons on why he wanted to play in Europe. And with a change of lifestyle and new surroundings, we are confident David will flourish with the Scarlets," he added.
Lyons had already packed a lot into his career. He recently became only the third player in Waratahs history to pass the magical 100 mark following in the footsteps of Chris Whitaker and Matthew Burke - 94 of which have been at Super Rugby level.
He is now back to full fitness after breaking his ankle playing for the Wallabies against Canada at the World Cup. Lyons' international career stretches back to the 2000, the same year he made his state and Super Rugby debuts. For a seven year period commencing in February 2000 he did not miss a match for the Waratahs in the Super 12/14 competition, an unbroken run of 83 games.
Lyons also achieved national recognition before playing a game of first grade rugby. He was born on June 15, 1980 in Orange, NSW, and grew up in the nearby town of Molong. Educated at Hurlstone Agricultural High School in Sydney's south west, he captained the touring Australian schoolboys of 1997/98. A representative of Sydney University at club level
As a result of Lyons capture, the Scarlets have secured a six-figure compensation fee with CA Brive Rugby for the early termination of Alix Popham's contract to allow him to take up an offer with the French club.
"I would like to thank Alix for his commitment and efforts on behalf of the Scarlets and would like to wish Alix good luck in his new career," added Mr Gallacher.
The 28-year-old Popham said: "I've had a great three years at Stradey Park and really enjoyed my playing career with the Scarlets. But this is a great opportunity for me and my family to experience a new culture and lifestyle. I would like to thank the fans who have been great to me and my family."
'Pops' has played 62 games for the Scarlets since making his debut against an Army Selection in August 2005. He has gained 33 Welsh caps and is hoping to add more in the near future.
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