The Brazilian is joining Atletico as a direct replacement for Peter Luccin, who is expected to join Primera Liga rivals Real Zaragoza.
Atletico coach Javier Aguirre had hoped to sign Motta's teammate, Rafael Marquez, but Barca refused to do business over the Mexican.
"I am very excited about playing for Atletico," said Motta last night. "I'll play wherever the coach wants me. I am ready."
Motta arrives as cover for central midfield, but can also play at centre-half for Aguirre if pressed.