The 20-year-old joins from Portuguese outfit Vitoria Guimaraes where he made his first-team breakthrough last term and will spend next season on loan at Getafe.
“Bernard Mensah is now an Atletico player after the agreement for his transfer was reached with Vitoria Guimaraes,” read an Atletico statement.
“The Ghanaian player has signed with our club for six seasons. He will play the first one with Getafe, on loan.”
Meanwhile, Moroccan goalkeeper Yassine Bounou will also spend next season away from the Vicente Calderon after agreeing to spend a second season on loan at Segunda Division side Real Zaragoza.