Dier has been a revelation for the Primeira Liga side after moving to Portugal at the age of 10 and joining their youth academy.
He made his first-team debut in August 2012 and featured in 13 league games last season as Sporting finished in second place to qualify for the UEFA Champions League.
However, Dier will not play in Europe's elite club competition after he agreed a deal to return to the country of his birth with Tottenham.
The 20-year-old will provide competition for places at the heart of Mauricio Pochettino's defence next season, fighting with the likes of Michael Dawson, Younes Kaboul and Jan Vertonghen.
Dier has seven England Under-21 caps to his name after making his debut against Scotland in August 2013.
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