Having spent the past season at English side Chelsea, Riley's involvement was limited to just 17 appearances, 10 of which came from the bench, after a foot injury in pre-season.
Confirming the news overnight, Chelsea manager Emma Hayes wished Riley all the best for her move to Germany.
"As the captain of New Zealand, and for a player her age, it's absolutely vital she's playing week in, week out," Hayes told Chelsea's website.
"We couldn't guarantee that with the squad numbers we have and the lack of Champions League football.
"She's made a telling impact on the Chelsea community, throughout the club - the amount of compliments I've received, from the media team, the education department, through to commercial.
"They've all said what a wonderful role model she is for the women's department. She's an advocate of a lot of positive news stories and I wish her all the best with her next career move. She'll be a huge success."