The Uruguay international midfielder has completed his £26m move from Sampdoria to the Gunners.
Emery said: “In Lucas Torreira, we have signed a young player who is a very bright talent in the game.
“A midfielder with great quality, I have enjoyed watching his performances for Sampdoria Uruguay World Cup.
“He is a young player already with good experience, but who wants to keep on growing.
"We welcome Lucas to Arsenal and look forward to him joining us soon for pre-season."