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.
"We are obviously massively disappointed to be losing a player of George's world-class quality and ability from Parc y Scarlets.
"However, we appreciate the reality that the profile and success he has achieved in such a short space of time attracts huge attention and makes him a prized player for any club in Europe.
"The last few weeks have been made increasingly difficult with considerable added pressure on George, those close to him and the Scarlets with the continuing level of intrusion, speculation and debate from outside the region surrounding George's future with us.
"We firmly believe that this situation illustrates that our policy of privacy for individuals is the correct and proper position for our business to take at all times.
"We fully respect the right of any individual to ensure they pursue the very best opportunities available to them, and now that a final decision has been reached, we sincerely hope George will be allowed to get on and enjoy the rest of his season with the Scarlets.
"He will leave with our sincere thanks for all he has done for Scarlets Rugby and our very best wishes from everyone in our region for the future."
Northampton's interest in North first surfaced shortly after the end of this season's Six Nations Championship, a tournament he helped his country win for a second successive title.
A row subsequently broke out between the Welsh Rugby Union and the Welsh regions after the WRU opted to bring North's situation into a public arena.
Although no figure has been released, it is thought that Northampton might have paid £200,000-plus to secure North's early release from the Scarlets.
North has so far not commented on the move.
But his representative Christian Abt said: "Once we had been informed by the Scarlets that a club outside the United Kingdom had made a significant approach, which the Scarlets had to consider, we focused on ways to keep George at the Scarlets, or within Wales.
"Given our initial intention to complete the existing contract, the Scarlets made the best offer they could for an extended agreement, but unfortunately, having taken into account every aspect involved in such an important decision, we were unable to agree terms.
"At the same time it became very apparent that due to a number of factors beyond our control and with no clear directive on the horizon, keeping George in Wales would prove increasingly difficult over time.
"Following the recent weeks of much publicised debate, we are delighted to have reached an agreement with Northampton Saints, who have made every effort to make what was a very difficult and emotional decision into a very sensible one.
"George can now look forward and work hard throughout the remainder of his season with the Scarlets before his move to Northampton in the summer."
Northampton chief executive Allan Robson added: "We are delighted to have reached an agreement with the Scarlets for George North to join us.
"We thank the Scarlets for their co-operation and responsibility in this matter since we made an approach via George North's representative to discuss bringing him to Franklin's Gardens.
"Everyone in our rugby community recognises and respects what an outstanding job the Scarlets have done to support, develop and invest in George North as a rugby player, along with many other internationals within their remarkably successful set-up.
"He is an exceptional talent and one of the most formidable back-three attacking players in world rugby.
"We look forward with great anticipation to what he can bring to his performances with Northampton Saints next season."
- More about: