We profile the teenager.
Unai Emery was full of praise for Arsenal's new signing. We take a look.
He made his first senior appearance for Lorient in Ligue 1 in October 2016 and played a bit-part role as the club were relegated in 2016-17.
Indeed, another midfielder, France u20 global Matteo Guendouzi is also set to join the Gunners.
Meanwhile, Arsenal have also signed another talented teenage prospect to their academy in 16-year-old striker Sam Greenwood. He is a France youth global, having represented them at under-18, under-19 and under-20 level.More news: Thai cave rescue: Four more boys extracted in second operation
Where would he fit at Arsenal?
Guendouzi caught the eye of Arsenal scouts and he's now admitted he's fulfilled a dream by joining the club he supported as a child.
"He is a young player already with good experience, but who wants to keep on growing".
It's something I'm immensely proud of.
However, the extent of Torreira's contract was not disclosed by the Premier League side.More news: Trump savors big reveal for Supreme Court pick
"It gives me great pride - it's incredible".
On Sampdoria's official Twitter account they have wished their former player well on his move to North London.
He added: "My hope is to become a top player here at Arsenal".
"We are delighted Matteo is joining us", said Arsenal boss Emery.
Torreira, who had been at Sampdoria for three years, said he did not feel nervous about the challenge awaiting him in playing at one of England's most historic clubs.More news: Liverpool Expect To Sign Xherdan Shaqiri In Next Two Weeks