The 20-year-old English centre-back has moved back to his homeland from Portugal, where he has spent the last 14 years
Spurs announced that they had agreed a deal for the 20-year-old on Thursday before confirming on Saturday that the centre-back had completed a medical and put pen to paper on a five-year deal.
And the Cheltenham-born defender is excited to make the move back to England and test himself in the Premier League.
He told the club's official website: "Obviously Spurs are a massive club. I've known about them since I was young, I've always enjoyed the way they play, the attacking football.
"I've always enjoyed watching them play, and coming from Portugal that suits me.
"This season I just want to adapt really quickly to the Premier League, do the best I can, try my hardest and hopefully get a good season."
Dier spent 14 years in Portugal, graduating from Sporting's academy and eventually breaking into their first team in 2013-14, an experience he relished.
"I moved there [Portugal] because of my family and we moved there when I was six," Dier added.
"I just got picked up by Sporting when I was eight and went through their youth system, all the way to the first team and just ended up playing in their first team.
"It's kind of a strange background, everyone seems surprised by it - an Englishman abroad - but I didn't go there because of football, it just happened that way really.
"I moved there with my family and football got involved in my life and went through Sporting's youth team like anyone would do here."