Atletico have struck a deal with Granada for the left-back, who agreed a four-year contract at Vicente Calderon last week and has now been unveiled in the Spanish capital after passing his medical.
Having signed for Granada in 2010, Siqueira spent last season on loan at UEFA Europa League finalists Benfica.
The 24-year-old is now looking forward to a new chapter in his career and had no hesitation in accepting Atletico's offer.
"I'm happy and excited," he said at his unveiling. "It is an important day because I can already say that I am at the Liga champions.
"This Atletico team is impressive, watching the team is a pleasure.
"Any player would want to be part of this group, it did not take a second to decide once I knew they were interested."
Siqueira's arrival at Atletico could pave the way for current left-back Filipe Luis to leave the club, having been linked with a close-season move to Chelsea.
‘I don’t think Liverpool would look at Ollie Watkins, a striker isn’t a pressing issue for them – it’s Arsenal who need one’ Former Reds star explains why his old club don’t need an out-and-out forward this summer
‘He’ll still be playing at 40 at a good level because he’s in such good shape and looks after himself so well. He does everything to be a top professional’: Ex-Liverpool coach insists Mohamed Salah has plenty more miles in the tank
