The news comes just over a fortnight after his brother Rory rejoined the Warriors after a four-year hiatus.
Sean, a powerful winger who can also play in the centres, spent two seasons in Scotland's number one city between 2003 and 2005, endearing himself to the Glasgow support with his fearless attacking and try-scoring prowess.
The 30-year-old crossed the whitewash 14 times in 32 RaboDirect PRO12 appearances, and established an even better European strike rate: seven scores in 10 games.
He moved to Northampton Saints in 2005, and has spent the last two-and-a-half seasons in Wales with Scarlets. He will see out the rest of the campaign in Llanelli before making his emotional homecoming.
Lamont, who has signed until the end of May 2015, told www.glasgowwarriors.org: "I'm delighted to be coming back to the Warriors. They're a fantastic club with a great team environment, a hugely talented squad and a very clear idea of where they want to go in the coming years.
"I've heard all about the plans to keep developing Glasgow on and off the field, and can't wait to have a look around Scotstoun Stadium, where we'll be playing our games as of next year.
"The club have clearly come a long way since I was last involved, and I'm thrilled at the prospect of playing my part in helping them write the next few chapters in the story.
"It will also be fantastic to be back at the same club as Rory. We both took huge pride in wearing that Glasgow jersey a few years ago, and it will be something really special to be pulling it on together once again.
"I'm pleased to have my future sorted out in such a positive fashion, but for now it's about putting the move to one side and giving my all for Scarlets during the rest of the campaign. I've thoroughly enjoyed playing for the club, and they've been great to me and my family."
Head coach Sean Lineen couldn't hide his delight at the announcement, saying: "I'm massively excited to have brought Sean back on board. He's a real tough competitor who knows his way round the RaboDirect PRO12 and Heineken Cup, and offers great positional versatility to boot.
"He's always kept Glasgow close to his heart, and that passion will be a fantastic asset for us when allied to his obvious playing and try-scoring ability.
"With David Lemi and Rory Lamont also having joined, we've now signed three fine international players in the last month. It's a real statement of intent from the club and Scottish Rugby, and it's now up to us as a squad and coaching staff to make the most of the tools we're being given."
Since making his Test debut in 2004, Sean Lamont has been a key figure in the Scotland side, starting all the team's games at the recent Rugby World Cup to take his cap haul to 60.
One of the most committed players on the circuit, his arrival is sure to be hailed by Warriors supporters who have watched their team suffer just a single defeat in 10 matches since 23 September.
Mark Dodson, Scottish Rugby chief executive, said: "This is another very encouraging piece of news for the Warriors and Scottish Rugby in general. Sean is an important part of the national team set-up who will be a real asset for Glasgow going forward.
"The Warriors have performed well so far this season, and we're delighted to support their efforts to keep improving."
Glasgow's next home game is the deciding leg of this season's 1872 Cup derby double header against Edinburgh Rugby at Firhill on New Year's Day (kick-off 5.35pm).
Tickets are available via www.glasgowwarriors.org, by calling 0844 800 3490, or in person at the Glasgow on Ice box office on George Square and the Gordon Street and Sauchiehall Street branches of Greaves Sports, our official retail partner.
Fans who book before midnight on Hogmanay will save £5 on the adult match-day price of £20. Students/senior citizens £10, under-18s £5.
Supporters should visit www.glasgowwarriors.org for details of a super saver joint ticket covering the Firhill derby and the crucial home Heineken Cup game against reigning champions Leinster, plus a raft of money can't buy incentives for clubs who sell derby tickets through their own websites.
Guinness PRO12
Suite 208, Alexandra House,
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Ballsbridge, Dublin 4, Ireland