The Finland international, 23, has signed a four-year deal at the Riverside after leaving Brentford for an undisclosed fee.
Forss helped Brentford win promotion to the Premier League during the 2020/21 season, when he scored eight Championship goals in 42 appearances.
The striker fell down the pecking order during the last campaign, though, when he was loaned out to Championship side Hull City in January.
"We're delighted to bring Marcus in," Wilder told Boro’s website.
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"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 has become Boro’s sixth new signing of the summer following the arrivals of Liam Roberts, Ryan Giles, Darragh Lenihan, Zack Steffen, and Tommy Smith.
But while Forss’ arrival will strengthen Boro’s attacking options, it is looking increasingly likely that attacking midfielder Marcus Tavernier will be leaving the club this summer.
Wilder and Boro were keen to keep the 23-year-old, who made 44 Championship appearances last season, but there has been strong interest from Bournemouth.
It’s now believed that Boro have reluctantly accepted an offer from the newly-promoted Premier League club for Tavernier.
As they did for Djed Spence, who joined Tottenham earlier this summer, the Teessiders will receive a sizable transfer fee if the move goes through.