"In Lucas Torreira, we have signed a young player who is a very bright talent in the game", Emery told the club's website.
A native of Fray Bentos, Torreira made his senior debut for Serie B side Pescara in 2015, signing for Sampdoria the following summer.
Lucas Torreira has signed for Arsenal from Sampdoria amd will wear the No 11 shirt.
"He is a young player already with good experience, but who wants to keep on growing".
The 5ft 6in holding midfielder flew to London earlier in the week to undergo a medical, and his acquisition will be seen as a huge boost to the club following the recent departure of Jack Wilshere to West Ham.
The Gunners thrashed out an agreement with the Italian Serie A side to seal the signature of the Uruguay global and he has arrived to bolster Unai Emery's options in midfield. His presence in the team will also help Aaron Ramsey and Granit Xhaka in elevating their game knowing that there is an assuring presence in the midfield.