Sources had told ESPN on Thursday the deal would be completed for €55 million ($65m), though Atalanta said it was an initial loan with an option to make the deal permanently in 2022.
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"I'm very happy to be wearing this shirt and to come to this club," Romero said in a video statement. "I want to thank the fans for all the messages. You wanted me here and here I am! Now I'll try and respond and repay all the faith that the club is showing in me. And I'm ready to start and give everything for this shirt."
Romero joined Juventus from Genoa in 2019 but returned to his former club on loan. In the following season, he moved to Atalanta on a two-year loan with the option to make the deal permanent for €16m.
The 23-year-old impressed at Atalanta as he helped Gian Piero Gasperini's side to a third-place finish in Serie A. Romero was named Serie A Defender of the Year at the end of the 2020-21 campaign.
Romero, who will wear the No. 4 shirt, is new Spurs boss Nuno Espirito Santo's third signing following the arrivals of goalkeeper Pierluigi Gollini, his former Atalanta teammate, and winger Bryan Gil from Sevilla.
Tottenham, who finished seventh last season, begin their Premier League campaign at home to champions Manchester City on Aug. 15.
Information from Reuters contributed to this report.