Sanchez has won 88 caps for Colombia and played in the Premier League for Aston Villa between 2014 and 2016.
West Ham United have endured relegation battles in their last two Premier League campaigns but striker Javier Hernandez says he is dreaming of a top six finish this season.
Sanchez's future appears to be away from Fiorentina after spending last season on loan at La Liga side Espanyol.
Perez said: "I'm very happy to be here at such an historic club".
The defensive-minded midfielder becomes the ninth player through the door at West Ham during a productive summer window for Manuel Pellegrini.
Lucas Perez: West Ham sign Spanish striker from Arsenal
Portuguese outlet A Bola, as quoted by Metro, is reporting that the north London side have deemed the 29-year-old to be surplus to requirements at The Emirates as Unai Emery prepares for his first season in charge at the club.
'I'd like to thank all of the people, the whole club, for having me'.
"I know that West Ham want him and they have made an offer", Lucas's agent Rodrigo Fernandez Lovelle told ESPN FC.
In a video posted a video to his Instagram story showing Perez wearing a West Ham training kit inside the dressing room.
"Lucas also has experience of the Premier League with Arsenal, and is familiar with life here in London, which of course means that he will take less time to adapt and be ready".More news: Saudi-Led Air Strike On A Bus In Yemen Kills 29 Children