Mats Hummels described the decision to leave Borussia Dortmund for Bayern Munich as the "most difficult" of his life.

The Dortmund captain's move to the Allianz Arena was announced Wednesday (AEST), with Hummels agreeing to a five-year deal to return to the Bundesliga champion.

Hummels, who left Bayern for Dortmund in 2009 after a loan spell at Signal Iduna Park, will re-join the Bavarian team on 1 July.

The 27 year-old has won two Bundesliga titles and the DFB-Pokal in his time with Dortmund and was part of the team that lost to Bayern in the 2013 UEFA Champions League final.

In a statement published on his Facebook and Twitter accounts, Hummels wrote: "After my transfer to Bayern was announced today, I would like to explain it in a little more detail.

"As you can imagine, the last few weeks have been very unnerving for me because I had to make the most difficult decision in my life. In the end I came to the decision to take on a new challenge for me, moreover, in my hometown, with my family and many friends that live there. I realize that it is difficult for some to understand my decision. But after eight and a half really great years in Dortmund it was time for me to try something new again.

"With a heavy heart I now leave the team and the club with which I have grown and was able to celebrate great success together with. So I thank all who have helped me. To you, the extraordinary fans of this great club, to all employees, the "behind the scenes" people. Of course, even with my three coaches that I experienced here and Michael Zorc and Hans-Joachim Watzke.

"And stressing even further to my teammates, because they – despite the fact that these last few years would not have been possible without them – always had an open ear for me. Of course I have seen many messages, comments, and postings hoping to convince me to stay at BVB. And also the great response at our home game against Wolfsburg, whether in the stadium, the training ground or in the supermarket, I was very touched.

"I speak for the day one BVB fans and I appreciate it very much."