"(Romero) signs for our club until June 30, 2031," said Atletico in a statement.
The Argentina international - who was also a member of the robust Argentina side that lost in last month's World Cup final to Spain - is Atletico's fourth signing this summer after Alejandro Grimaldo, Kang-in Lee and Morten Hjulmand.
The 28-year-old said goodbye to Spurs fans on Friday after five years in north London.
He was named Man of the Match when the club ended a 17-year trophy drought beating Manchester United to win the 2025 Europa League.
Romero, who incurred several suspensions during his 156 appearances for Spurs, acknowledged "the journey wasn't perfect" in his leaving message, but said he was departing with a "heart full of memories" and "enormous pride".
Romero arrives days after compatriot and fellow World Cup winning defender Nahuel Molina left Atletico for Serie A side AS Roma, for a fee reported to be 17 million euros (£14.5m).
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