Darren Pratley has decided to leave Swansea City to seek a new "challenge".
The 26-year-old was out of contract at the end of the season, and BBC Sport Wales understands he rejected an offer of a new deal to stay in south Wales.
Midfielder Pratley, a £100,000 signing from Fulham in 2006, recently denied he had agreed to join Bolton Wanderers.
"I'd like to thank the chairman [Huw Jenkins], manager [Brendan Rodgers] and especially the fans for a wonderful five years," said Pratley.
"But after five years I feel the time is right for a fresh challenge."
Pratley helped Swansea win two promotions in his five years in south Wales, but negotiations on a new contract were put on hold throughout last season.
The energetic midfielder lost his starting place in the latter part of last season as Swansea secured a play-off place by finishing third in the Championship.
Swansea rejected a bid from Forest for Pratley in August 2010, but a career-best return of 10 league goals from 28 starts last season caught the attention of Bolton, Wigan Athletic and Newcastle United.
"It's been a difficult decision for me and my family to make because Swansea is a fantastic club with such friendly and passionate supporters," said Pratley.
"It's not going to be easy for me, but if there was a good time to leave then it's now with the club having secured their place in the Premier League.
"I hope people feel I have contributed to that rise from League One and my goal in the play-off semi-final in front of the Swansea fans was a nice way to finish at the Liberty.
"I wish the club, the players and the supporters the very best of luck in the Premier League and they will always be in my heart.''
Swansea chairman Jenkins, who is in contract talks with goalkeeper Dorus de Vries and defender Angel Rangel, paid tribute to Pratley's contribution.
"As a club we would like to thank Darren for his services over the last five years,'' Jenkins said. "He has played a key part in our success over the years and while we are sorry to see him go we wish him every success in his new challenge.
"He has always been a good professional and the door will always be open with a warm welcome awaiting him at Swansea City.''
Swansea, who broke their transfer record by signing Danny Graham on Tuesday for £3.5m, face losing full-back Neil Taylor after Newcastle made a £1m bid this week.
The club are also poised to fend off interest for centre-back Ashley Williams, with Queen's Park Rangers having tabled a reported £2.6m bid.
Meanwhile, Dutch winger Cedric van der Gun has been released by Swansea, along with defender Albert Serran, and youngsters Kerry Morgan and Jamie Grimes.