The 30-year-old former Celtic captain becomes Motherwell's sixth signing of a busy summer.
The Steelmen have already snapped up Paul Lawson, Iain Vigurs, John Sutton, Fraser Kerr and Gunnar Nielsen, and there are reports circulating that James McFadden may be set to re-join the club.
McManus has 26 Scotland caps and played over 200 times for Celtic between 2003 and 2010, before joining Boro in 2010.
He is excited about his return to Scottish football and told MFC TV: "I am really glad to get it done and dusted.
"There was a fair bit of interest from the manager (Stuart McCall) early on, so I am pleased it has now been finalised.
"From the minute I came in the door for the first time, there was a real family atmosphere and that is what drew me to the club.
"I always had an idea then that I was going to come here, even though it was a hard decision given other clubs were interested.
"I just can't wait to get started. I will give it my best shot and we will see how it goes.
"We have a really good squad and the manager will probably add some more new faces too."