Perez has signed a three-year deal with the Hammers and Sanchez moves to the London Stadium on a two-year contract.
The 32-year-old Colombian becomes Manuel Pellegrini’s ninth summer signing since taking charge. West Ham has signed Spanish striker Lucas Perez, 29, from Arsenal for a reported £4m, and Colombia midfielder Carlos Sanchez from Fiorentina.
West Ham kicks off their Premier League campaign with a trip to Liverpool on Sunday, 12 August.
Perez, the second former Arsenal player to join following Jack Wilshere’s arrival, scored seven goals in 21 appearances for the Gunners having joined them from Deportivo La Coruna for £17m in August 2016.
The former Rayo Vallecano and PAOK player spent last season on loan back at Deportivo, scoring eight times in 35 La Liga appearances.
“I’m very excited, as West Ham are giving me the chance to stay in the Premier League,” Perez told the club’s official website.
“I’m very happy to be part of this new project. West Ham has great fans and play in an amazing stadium, so to sign here brings me great joy. I’m very motivated.”
West Ham director of football Mario Husillos added: “He is a very experienced player that Manuel and I both know well from Spain.
“He has great technical quality and can play as both the main striker or the second striker in a deeper role. He scores goals but can also create them and link well with the midfielders.”
Defensive midfielder Sanchez, who has won 88 caps for his country, said he was “keen to achieve good things” with the Hammers.
“I decided to come here because, first, it is the Premier League, second, it’s a beautiful club and the fans are always helping the club and, third, for me it’s an amazing opportunity and an amazing project,” he added.
Husillos added: “He has great quality as a holding midfielder. He can also cover other positions but it is as a defensive midfielder that he really excels.
“I have known the Colombia national team coach, Jose Pekerman, for many, many years, and he has always spoken very highly of Carlos.”