Tomlin completed a medical earlier on Tuesday and becomes the Cherries’ ninth signing of the summer.
Boro paid £1.5m to bring the 26-year-old to the Riverside Stadium in January 2014 but he has barely featured in manager Aitor Karanka’s plans during pre-season.
Tomlin scored 10 goals in 50 appearances last season and will now be given the opportunity to impress at a higher level as Eddie Howe’s side try to ensure their stay in the Premier League for an extended period.
“Lee is a player who we are excited about and will get fans off their seats,” chief executive Neill Blake told Bournemouth’s official website.
“The promotion race in the Championship last season was incredibly competitive and Lee was a key figure in a Middlesbrough side who went very close to going up.
“We believe he is ready to make the step up and look forward to seeing him play in the Premier League.
“Like the club, he has had to work hard to get to this point and is eager to make the most of this opportunity.
“We are very pleased with the business we have done this week, and on the whole this summer, and feel it is a marker of our ambitions this season.”
Tomlin began his career at Rushden and Diamonds and also enjoyed a successful spell with Peterborough before making the move to Teesside.