Nick Compton has signed a long-term contract with Somerset after Middlesex agreed to release him.
Compton, 26, was Middlesex's top run scorer last season with over 1,600 in all competitions, including five centuries.
The Durban-born right-hander, grandson of England legend Denis Compton, was only one season into a new deal at Lord's.
But he also had a verbal agreement with Middlesex director of cricket Angus Fraser allowing him to leave at the end of the 2009 campaign - if he so desired.
And Compton has exercised his right to depart the Londoners in order to move to Somerset, who contended in all competitions last season without picking up any silverware.
"I am very excited about joining Somerset, a county I have always had great respect for," Compton told the county's official website.
"I would like to thank them for sharing their vision and it was that vision that made my decision an easy one. I look forward to being part of the reason we achieve our goals at Somerset next year and beyond."
Somerset director of cricket Brian Rose views Compton as an ideal replacement for Justin Langer, who has retired after three seasons in charge of the Cidermen.
"Nick is a player we view as having the ability to bat in our top three and be with us for many years," Rose said.
"With Justin Langer leaving, we needed to strengthen our batting options.
"This summer he had a very strong season, particularly in one-day cricket, and he was also solid in the Championship. He will join our crop of talented young players such as Jos Buttler and Chris Jones.
"He has mainly opened in his career, but he could bat at three or Arul Suppiah could drop down. That is an issue for our new captain Marcus Trescothick, who rates Nick highly."
Trescothick, who has succeeded Langer as skipper, added: "I am delighted with the signing of Nick Compton, who is a good player in four-day and one-day cricket.
"After a hugely promising start to his career, he encountered the problems a lot of batsmen, including myself, have gone through as bowlers start to work you out.
"But he seems to have come through that period now and I believe he will be a big asset."