Romero had already announced his Spurs exit on Friday as he penned an emotional goodbye to Tottenham fans on social media.
Now, Atletico have announced his summer move in a £34M deal which will allow Tottenham to reinvest in defensive reinforcements.
The Argentina international spent five years in north London and now swaps life for the Spanish capital as he aims to fight for silverware with an Atletico side who finished 4th in La Liga last season.
Speaking on the switch, Romero highlighted his ambition for silverware and why he thinks the move is perfect for him.
"I am very happy, very delighted to arrive here, at this huge club. I know I come with the same mentality as always, trying to write my name in the history of this beautiful club."
"I know the only way is by winning trophies, so I come with that mentality and I will give everything until the last day to achieve it. It's a club that, because of my style of play, fits me perfectly.
"There are many national team-mates here, and I know it's a wonderful club. When I played against them in the Champions League, the fans supporting them... a bit of everything made me make this decision to come here. I will give my absolute all, no doubt. In the end football can go well or bad, but I know I'll be 100 per cent committed to this crest, this shirt."
He will join up with manager and compatriot Diego Simeone at Atletico, with the Argentine manager a long-term admirer of Romero.
"I speak from a point of view as an Argentina fan. I admire his personality, his character, his defensive work and his football ability on the ball," Simeone said earlier this year.
"As a fan of Argentina, I have seen him play more games for the national team than I have for Spurs, but I'm a big fan of him, for sure."
Pedro Porro and Micky Van de Ven are being tipped to take over as Tottenham captain after Romero’s departure. With just one week before the Premier League season kicks off, Spurs must move quick to establish leadership in the side or risk a shaky start under manager Roberto De Zerbi.