Friend becomes Tony Mowbray's sixth summer signing following the arrival of Grant Leadbitter, Stuart Parnaby, Christian Burgess, Jonathan Woodgate and Emmanuel Ledesma.
Boro beat off competition from two other Championship clubs to secure the highly-rated Doncaster captain, who was named Player of the Year and Player's Player of the Year at The Keepmoat Stadium last season.
"I think George will be a good signing for us," says Mowbray. "He's a versatile player who can play at centre-half and left-back. He's very athletic and very mobile and has all the attributes you look for in a player.
"He's going to create really good competition to get into our side."
Friend believes his move to Boro will boost his chances of playing in the Premier League again. He savoured a brief taste of the top flight with Wolves as a youngster and is anxious for more.
"Middlesbrough finished just outside the play-off places last season and Tony Mowbray is building a really strong squad here," he says.
"I took that into account when deciding to move here because my ambition is to help this club achieve promotion.
"It has been fairly crucial to make then right choice, and a big attraction was how the manager (Mowbray) likes to play football. I've seen what he has done at previous clubs and his teams like to pass the ball. "I hope I fit into that category and I can't wait for the season to start.
"The manager has made some quality signings this summer and these are exciting times for Middlesbrough."
Barnstaple-born Friend started his career at Exeter City and helped the West Country side regain their Football League status with a Conference play-off triumph over Cambridge United at Wembley in 2008.