Assuncao has gained a release from his contract at Atletico.
He said onclubatleticodemadrid.com: "I've experienced the best moments of my career at Atletico, winning three European titles.
"I'll always remember my time at Atletico. I hope they do very well this season and achieve their objectives."
He added: "I'm really excited about returning to Brazil and hope I can continue showing what I can do."