Wales wing George North will join Northampton this summer on a three-year deal.
The Scarlets announced they have agreed to release North a year early from his contract to move to Franklin's Gardens.
The Scarlets say an agreement "between all parties" was reached late last Friday.
North, 20, will become the latest high-profile Wales international player to quit the Principality when he departs for Northampton.
The likes of his RBS 6 Nations title-winning colleagues Mike Phillips, James Hook and Gethin Jenkins all currently play in France, while centre Jamie Roberts and flanker Dan Lydiate are expected to join wealthy Parisians Racing Metro this summer.
North will follow prop Paul James into the Aviva Premiership. James moved to Bath last year.
North, who made his Test debut as an 18-year-old in 2010, has won 31 caps and is rated a certainty to go to Australia with the British and Irish Lions in June and July.
Scarlets chief executive Mark Davies said: "Everyone at the Scarlets will wish George the very best in his future career now that it is clear he will be playing his club rugby in Northampton for the next few years.
"Despite our desire to retain all our internationals within the region, we have to balance our available resource over the need to sustain a 50-deep player squad that can continue to compete during international periods.
"Taking that into account, we made George the very best offer we could. However, we have to fully understand and appreciate that George's quite unique value in the rugby marketplace, possibly inside Wales, but certainly outside Wales, is considerably greater than the Scarlets as an independent business can reach."