The 28-year-old will join the La Liga club for a reported £34.2million fee after spending four seasons with the North London side and helping them to win the 2025 Europa League title.
Romero had previously been linked with Inter Milan, Barcelona and Spurs arch-rivals Arsenal during the summer transfer window but is now destined to play under Argentine head coach Diego Simeone at Atleti next season.
Tottenham have also reportedly included a clause in the deal to receive 15% of any future transfer of the centre-back.
Atletico have already made three major signings this summer, with midfielder Morten Hjulman joining from Sporting Lisbon, attacking midfielder Kang-in Lee from Paris Saint-Germain and left-back Alejandro Grimaldo from Bayer Leverkusen.
The new Spanish La Liga season begins this weekend, with Los Che set to play their first fixture against Malaga next Wednesday.
