Foyth had been with Estudiantes his entire career having joined the clubs youth ranks and has progressed through the system to become a first team regular over the past year.
Juan Foyth established himself in the Estudiantes' first team and has since represented Argentina at Under-20 level 12 times.
Foyth, an under-20 Argentina worldwide, was linked with both Tottenham and Paris-Saint Germain in July and at one stage it seemed like Mauricio Pochettino would miss out on the centre back.
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Chelsea are keen on strengthening their English core after fielding no players from the country in the last two fixtures. The fee for the midfielder, who has three England caps, is likely to top £30 million.
"It's a club that believes in young players and I'm very pleased". I'm focused now and I'm going to try and make the most of it. It's a dream for me.
"My agent knows what leagues attract me and Russian Federation is not one of them in this moment", he told Radio Continental.
The teenager becomes the third player to secure a move to the North London club after defender Davinson Sanchez and goalkeeper Paulo Gazzaniga both completed deals to Spurs earlier this summer.
Spurs have been searching for defensive recruits throughout the summer and the club have been heavily linked with a move for Foyth, who was also attracting interest from Paris Saint-Germain, throughout the window.