The official announcement was made today (Thurs) after Lamont cleared a medical undertaken by the Scarlets this week - standard with all new player contracts.
He said: "I'm thrilled to be joining the Scarlets. They're building an exciting squad and are based at one of the finest rugby facilities I've ever seen.
"There's great passion for the sport in Wales - especially across the Scarlets region - so I can't wait to get going and join up with a great group of players and such a well-known rugby club with proud traditions.
"I'm determined to make a major contribution on and off the field. The Scarlets supporters can expect a massive amount of commitment and dedication from me.
"I've had four good years at Northampton and will be leaving behind a lot of good friends. I thank them for all they've done for me and hope I've left the fans with some good memories.
"But I'm now ready for a fresh start and a new chapter with the Scarlets at this imposing stadium and look forward to making an impact."
Scarlets head coach Nigel Davies said: "We've had our eye on Sean for some time; he'll bring pace, great power and considerable skill to our backs unit.
"He's a man of experience, with a lot of game time in the international arena, and that can only be a good thing for our squad as we look to bring on and develop our own up-and-coming Welsh players.
"I have said repeatedly that we want to revitalise and balance this Scarlets squad for the long-term and that means bringing in some top end quality players in the right positions, where we need strength and experience - as well as backing and developing some exceptional emerging Welsh talent.
"Sean Lamont is a seasoned professional - a top-class international with a proven record and has been playing really well for the Saints recently. He fits well into our set-up, will have an impact on the field and help raise the game of some of our younger players. We are really pleased he is joining us."
In 2007 Lamont won the Scotland player of the season award. In 20 club appearances this season, he has touched down six times. One of those tries came in November's EDF Energy Cup victory over the Scarlets at Franklin's Gardens. His career total for the Saints is 23 tries in 68 games.
His international career has brought seven tries, his latest cap coming against Wales in this season's Six Nations in February 2009 at Murrayfield. He played in four of Scotland's five 2007 Rugby World Cup matches and has also represented Leicestershire Colts, Scottish Exiles and Scotland A.
Lamont, who stands 6ft 2ins and tips the scales at 15st 10lb, was born in January 1981. From school in Leicestershire he went to Sheffield Hallam University to study sports science from 1999-2002.
While there in 2000 he joined Rotherham RFC and was the club's U21 captain in 2001. He played sevens for Scotland at the 2002 Commonwealth Games and joined Glasgow Rugby in 2003, scoring 12 tries in his first season and being named player of the year.
His first cap came on tour against Samoa in 2004, he was man of the match against Italy in the 2005 Six Nations and scored two tries as Scotland beat France in the next championship.
By then he was with the Saints, an early highlight being a four-try haul in one 2006 Guinness Premiership game.
Sean is married with two sons.
Sean Lamont fact file
DOB - 15th Jan 1981
Birthplace - Perth, Scotland
Height - 1.88 m (6ft 2in)
Weight - 100 kg (15st 10lb)
Position - Wing
Honours - Scotland (37 caps)
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