As per the club’s announcement this afternoon, the 23-year-old has agreed to a four-year deal.
Boro boss Chris Wilder has made it no secret in recent weeks that he was looking for additions in the striking department, and is clearly happy to get this one over the line ahead of the Championship’s opening weekend.
“We’re delighted to bring Marcus in,” Wilder told Middlesbrough club media.
“He played a big part in Brentford winning promotion a couple of seasons ago, and he knows this league well.
“He’s a good athlete and all-round player.
“We’re really looking forward to what he can bring to the squad.”
Forss becomes the sixth signing of the summer for Wilder and the club, but, crucially, the first in the forward area.
Elsewhere, Darragh Lenihan, Tommy Smith, and Liam Smith have arrived at the Riverside on permanent deals.
Meanwhile, Zack Steffen and Ryan Giles have both arrived at the club on loan deals, with the former from Manchester City and the latter, Wolves.
Quiz: 20 statements about former Middlesbrough players – Are they true or false?
Boro begin their 2022/23 campaign with a home match against West Brom on Saturday evening.
Kick-off at the Riverside Stadium is scheduled for 17:30PM, with the match also being broadcast live on Sky Sports in the United Kingdom.
This is an interesting signing for Middlesbrough.
Whilst his arrival is very much welcomed given the state of their front-line at present, you still feel that the club need at least one more signing in the area.
Nevertheless, given that he is fit enough to do so, you would assume that Marcus Forss would go straight into the starting line up for the clash against the Baggies.
Having shown real glimpses of his ability at Brentford, Forss himself will be hoping that having found a new permanent home, he can put a brief, disappointing, loan spell at Hull City last term behind him.