The 24-year-old defender was previously with Club America in Liga MX, where he won a league title in the 2018 Apertura and a Campeon de Campeones trophy in 2019.
Last Saturday, America announced that they had come to an agreement with Ajax for Sanchez, who traveled to Amsterdam over the weekend to complete the move.
"[I'm] motivated because going to Europe is not so easy. I'm thinking about my future and about the Mexican national team," Sanchez said before leaving for Ajax, calling the move a "golden opportunity."
The Santos Laguna academy product's deal is for four years, keeping him at Ajax until 2026.
Since his national team debut in 2019, Sanchez has been a regular call-up for manager Gerardo "Tata" Martino. Ahead of the 2022 World Cup -- where Mexico will face Poland, Argentina and Saudi Arabia in the group stage -- the defender could solidify a starting spot with El Tri through a strong showing to begin the Eredivisie season.
Ajax, the current Eredivisie title holders, began their new season with a 3-2 away win over Fortuna Sittard last Saturday. On Aug. 14, they'll host FC Groningen in a home-opener at the Johan Cruyff Arena.
On Aug. 25, Ajax will find out their UEFA Champions League group stage opponents during the draw in Istanbul.