BVB tweeted Hummels' return on Wednesday:
Hummels left Dortmund for Bayern in 2016.
Goal's Ronan Murphy relayed the details of his return:
The centre-back began his career at Bayern but left for Dortmund after making just two senior appearances.
He had plenty of success with BVB, too:
He returned to Bayern as a top-class defender and helped them win three consecutive Bundesliga titles.
His form has dropped a little of late, though. He made just 21 league appearances last season, and Bayern have recruited Lucas Hernandez and Benjamin Pavard, both of whom can play centre-back, this summer.
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Bleacher Report's Sam Tighe believes Hummels could still be a strong addition back at Dortmund:
The 30-year-old World Cup winner is a leader and has an enormous amount of experience, which will be valuable qualities in a young side.
BVB finished second to Bayern by just two points, but they shipped 44 goals to Bayern's 32, so the defence was clearly an area in need of improvement.
As football writer Jonathan Harding noted, not all the fans will be best pleased by his return, though:
If his added defensive quality can help the team beat Bayern to the title next season, it's likely all would be forgiven on that front.
Winning the title will be a challenge, but Hummels' arrival should strengthen their chances of doing so.