Tomlin became the Cherries' ninth signing of the summer after completing his medical on Tuesday and his arrival has been warmly welcomed by chief executive Neill Blake.
"Lee is a player who we are excited about and will get fans off their seats," 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. “
Tomlin scored 10 goals in 50 appearances for the Boro last season.
BREAKING: #afcb sign @Leetomlin10 from #boro on a three-year deal: http://t.co/fHUshOIharpic.twitter.com/67Tu6aHGfz
— AFC Bournemouth (@afcbournemouth) August 4, 2015