The Slovenian will fill the boots of departed Chelsea keeper who has been the main man for the club for three seasons on loan, the club revealed on Wednesday.
Jose Mourinho has publicly insisted that the Belgian shot-stopper will not be loaned out for a fourth successive season to the Vicente Calderon by the Blues and the signing of the Slovenian goalkeeper suggests that Diego Simeone is ready to move on.
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The 21-year-old was part of Benfica's treble-winning team last season, making 25 appearances between the sticks for Jorge Jesus' men, and is reported to have set La Liga's holder back a hefty 16 million euros.
"Atletico Madrid and Benfica have reached an agreement for the transfer of Jan Oblak to our club, pending medical examination and the physical signing of the contract by the player," the club stated on its website.
"The Slovenian keeper will commit for the next six seasons."
Atletico sporting director Jose Luis Perez Caminero is confident that Oblak - who has three senior Slovenia caps to date - has one of the brightest futures of any young shot stopper on the planet.
"He is a goalkeeper with great quality and a great future, which will benefit us greatly," he said.
"He is one of the goalkeepers with the best potential in the world and we are very happy with to have him. This is great news for our club."
Oblak joins fellow new goalkeeping arrival Miguel Angel Moya in the scrap to fill Courtois' boots for Atletico in 2014-15.