Goal revealed on Thursday that Spurs were on the brink of completing a £4 million move for the Englishman, which is believed to be a five-year deal.
And Spurs confirmed the deal on Thursday evening, posting a message on their official Twitter account.
Dier has represented England at Under-19, Under-20 and Under-21 level and made 15 appearances for Sporting last season.
Born in Cheltenham, Dier has spent his entire career at Sporting, but enjoyed a short loan stay at Everton's academy in 2011.
He will compete with Michael Dawson, Younes Kaboul, Jan Vertonghen and Zeki Fryers for a spot in Mauricio Pochettino's defence this season.