Quinnell told Wales on Sunday: "You get to the stage where you know enough is enough. I'm at that stage and now I've got to look at new opportunities. I've given all I can to the playing side.
"The game's getting tougher, I've had a lot of injuries over the years. My knee's been playing me up for six or seven years.
"My physiotherapist will probably be glad I'm retiring, it will cut his time by half!"
The announcement undoubtedly heralds the end of an era - but not quite yet. Scott is keen to point out that there's a long way to go this season and he has every intention of ending on a high with some typical barnstorming performances. "I'm looking to the long term but my focus for now will be on helping the boys turn things round and have a much better second half to the season."
Scarlets coach Gareth Jenkins said: "We'll be sad to see him go next year but first Scott has got a big part to play in the rest of the season and our ambition to qualify for next season's Heineken Cup. I really hope he enjoys these games and knowing Scott he'll pull something special out of the bag."
British and Irish Lion Quinnell, who called time on his international career in 2002 against Canada - nearly nine years to the day after he made his debut against the same opposition - says he will make the most of his last months in the game.
Quinnell continued: "I want to give 110 per cent to the game and look forward to what's beyond that," he said. "It's going to be a very sad time - I've been playing rugby since I was eight, when I started at Stradey Park.
"I think it's going to be worse than when I retired from internationals. It's the end of a huge chapter in my life. It's a new beginning, but what that is yet, I just don't know.
"There's a long way to go and I'm just going to take every game as it comes."
Quinnell, who has been capped 52 times by Wales, captaining his country on seven of those occasions, as well as take part on two tours with the Lions in 1991 and 1995, will concentrate on coaching next season.
In 2004 he has combined his playing duties at the Scarlets with a role as a part-time coach of Llanelli's semi-professional side.
"It's been wonderful to have the opportunity to coach the Llanelli team this year. I would love to have the opportunity to coach afterwards, but unfortunately there aren't many jobs in Wales.
"I know there's a couple of English clubs interested, but I would prefer to stay in Wales and put something back into the community."
Quinnell's decision follows the news in November that his teammate with Wales and the Lions, scrum-half Rob Howley, was forced to retire from rugby due to injury.
Quinnell, whose brother still plays for Cardiff and played for Wales, comes from a strong Welsh rugby family. His father Derek also played for Wales and his uncle is another all-time great, fly-half Barry John.
He has been a key figure for Llanelli in their Heineken Cup exploits, playing on 34 occasions and scoring 11 tries.
Chief Executive, Stuart Gallacher, added: "Scott's performances for us, particularly at the highest level of European competition, have been outstanding. But we'll also miss his qualities off the pitch where he has done so much work in the community."