The 21-year-old recently already announced that he would leave Werder after just one year at the club and he has now inked a contract with Bayern until June 2020.
Gnabry had also attracted the interest of Hoffenheim and RB Leipzig with his fine performances in 2016-17, but the reigning Bundesliga champions have won the race for his signature.
"It is a big honour to become a part of Bayern," Gnabry commented on his transfer.
"I have an exciting challenge ahead and I am really looking forward to it."
@SergeGnabry #Gnabry: "It's a huge honour for me to sign for #FCBayern. I have exciting times ahead of me, which I look forward to." #MiaSanMia pic.twitter.com/3SjyOPXyjS
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