The 24-year-old has played 67 matches and scored 420 points for the West Wallians since arriving from Leicester Tigers in 2020.
Costello, who has been capped 18 times for Wales, recently returned from a shoulder injury to help steer the Scarlets to a stunning victory over URC leaders Leinster last time out.
Looking to the future and making his mark, over the new few seasons, Costello said:
“I am really grateful to the Scarlets for giving me this chance.
“We have a good group here, we’re ticking along nicely and it is a place where I want to be right now. We have had a lot of learnings this season, but it has been a good year.
“It is never nice to get injured, you want to play as much rugby as you can, but being back out there with my mates against Leinster, playing in front of the fans again, it was awesome.
“The support we get here means a massive amount to us as players. I know it is a cliché, but they are our 16th man with their singing and chanting and it really does give you a boost, creates a good feel and gives us energy on the field.”
Scarlets head coach Dwayne Peel said:
“It is fantastic that Sam has signed a new deal. He is a leader in our group, he has a presence on and off the field and it was great to see him back against Leinster after four months or so out with a shoulder injury.
“We all know his quality, he is still only 24, but he has a lot of experience already, both for us and at international level. He is going to have a big part to play in this side over the coming seasons.”
The latest re-signing news comes off the back of the announcement that scrum-half Dane Blacker is returning to the Scarlets this summer from the Dragons, along with former lock Jake Ball.
The Llanelli-based region have also signed Wales centre Joe Hawkins, and have seen numerous first teamers pen new terms in the last few weeks.
[Lead image: Scarlets Rugby]
