The London club revealed on Thursday that they had agreed a deal for the 20-year-old centre-back and he has now put pen to paper on a five-year deal.
Portuguese giants Sporting stated that Tottenham had triggered a €5 million release clause in Dier's contract, but the Premier League club have not confirmed the fee paid for the youngster.
Dier joined the Primeira Liga side after moving to Portugal at the age of 10 to join their youth academy, and the Cheltenham-born defender is relishing the prospect of returning to his homeland.
He told Tottenham's official website: "I'm really excited to be joining such a big club in the Premier League.
"It's an amazing place (White Hart Lane). I've just been up on the balcony and seen the pitches - it looks fantastic. I'm really looking forward to getting started."
Dier made his first-team debut for Sporting in August 2012 and featured in 13 league games last season as the Lisbon outfit finished in second place to qualify for the UEFA Champions League.
He also has seven England Under-21 caps to his name after making his debut against Scotland in August 2013.
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