Mats Hummels has signed a five-year deal with Bayern Munich after passing his medical on Monday.
Bayern announced earlier this month that Borussia Dortmund captain Hummels had agreed to swap Signal Iduna Park for the Allianz Arena, and the transfer has now been completed.
Hummels commented: "Saying farewell to Dortmund has not been easy, just like it was not easy when I moved in the opposite direction in 2008.
"I am now looking forward to everything that awaits me at Bayern. I am very relaxed."
Bayern chairman Karl-Heinz Rummenigge added: "We are very pleased to be able to welcome back Mats to Bayern.
"We have not only added a world-class centre-back to our squad with the capture of Mats, but also someone who does not shy away from taking his responsibility and who came through our youth academy.
"We now wish him a successful Euro 2016 first of all."
Germany centre-back Hummels joined Bayern at the age of six in 1995, only to leave for Dortmund in January 2008.
His final game for Dortmund came against Bayern in the DFB-Pokal final on Saturday, where he was substituted after 78 minutes due to injury before the Bavarians won on penalties.