Hummels returns to Bayern after eight and a half year at Dortmund.
He won two Bundesliga titles and the DFB Cup with Dortmund.
Says it was a difficult decision, but needed a new challenge.
Mats Hummels has written a letter to his fans after his transfer from Borussia Dortmund to FC Bayern München was confirmed.
Here is what the German central defender wrote on his Facebook page:
"After my transfer to FC Bayern was announced today, I would now like also to go into some more detail about it.
As you can imagine, the past few weeks have been very nerve-racking for me too, since I had to make the most difficult decision of my life so far. In the end, I came to the realisation that I wanted to accept a new challenge, one in my home city, where my family and many of my friends live.
I know that it will be hard for many of you to understand my decision. But after eight and a half really amazing years in Dortmund, the time had come for me to try something new.
It is with a heavy heart that I now leave the club and the team with whom I have grown and been able to celebrate great successes. Therefore, I would like to thank everybody who has made a contribution to this; you, the extraordinary fans of this great club; all of the employees who have done their work 'behind the scenes'. Of course I would also like to thank the three coaches I have worked under here, and [sporting director] Michael Zorc as well as [CEO] Hans-Joachim Watzke.
And particular thanks go to my team-mates since you – irrespective of the fact that these past years would not have been possible without you – were always there for me.
Of course I have seen all the many messages, comments and posts aimed at convincing me to stay at BVB. And the great encouragement in our home game against Wolfsburg, whether it was in the stadium, on the training field or at the supermarket, touched me profoundly. This just goes to show how unique BVB fans are, and I really appreciate it a lot.
Now I will give everything in these remaining training sessions and matches to honour my time here in a worthy fashion: by coming back to Dortmund with the DFB Cup in our hands!