Eddie Howe has left Burnley to join Bournemouth after the two clubs agreed a compensation package for the manager.
Howe previously managed the Cherries for two years and kept them in the Football League before winning promotion in 2010.
"It's been a really tough week," he told Burnley's website.
"To leave is incredibly difficult, but I feel it's a decision I have to make for my family and for personal reasons, which I can't go into detail on."
Howe, whose mother died in March, added: "There is only one club I would contemplate leaving Burnley for and that is Bournemouth and that is the situation we found ourselves in."
His return is a major coup for ambitious Bournemouth who sacked previous boss Paul Groves on 3 October.
The 34-year-old, who played more than 250 times for the Cherries, had been in charge of Burnley for nearly two years.
But having joined in January 2011 he has failed to make a significant impact at Turf Moor.
On Thursday, Championship club Burnley confirmed they had received an "unwanted approach" for Howe, which was later confirmed to BBC Radio Solent by Cherries chairman Eddie Mitchell.
Howe had been linked with a switch to Bournemouth last week but refused to comment on the speculation.
And after announcing his departure back to the Dorset club, Burnley confirmed on their website that Howe's exit was due to "personal and family reasons" but joint chairmen John Banaszkiewicz and Mike Garlick said they had not expected him to leave.
"We are surprised and disappointed that Eddie is leaving a great club like Burnley so soon in the season," a statement from the pair read.
"However, we understand that his decision reflects his personal circumstances rather than any matters at Burnley Football Club, be that supporters, the players or the board."
Burnley have made a mixed start to the Championship season - they lie 16th in the table with 11 points from their first 10 matches.
Howe, who was raised in Dorset, won 51 of his 102 matches in charge of his hometown club and kept them in the Football League in his first season in charge in 2009. The following season they were promoted to League One.
Burnley's assistant manager Jason Tindall, who was also Howe's right-hand man at Bournemouth, has also joined him in moving to the Goldsands Stadium.