The versatile 22-year-old has already appeared for the Frauen-Bundesliga side close to 100 times and will complete her move at the conclusion of the current campaign.
Nusken, who has also won 15 caps for Germany and scored two goals, said: “I wanted to move to the Women’s Super League because it’s a good league, English football is very cool to play in and it’s the right thing to do now.
“I’m very excited and happy to be at Chelsea and I’m looking forward to next season.
“It’s a big club with such good players. I hope to improve my football skills. I’m very excited to meet my new team-mates and I can learn a lot from them.”
Chelsea manager Emma Hayes said: “Sjoeke is a fantastic young midfield player whose ability to play the ball deep into the opponents’ half is not her only standout strength, there is everything from her interceptions, her reading of the game and her overall presence. She’s dynamic, she’s good aerially and she’s going to be a fantastic signing for this club.”
Chelsea won the Women’s FA Cup for the third successive year on Sunday, beating Manchester United 1-0 in the final, and are in the running to claim a fourth consecutive WSL title.
They can move two points clear of current leaders United at the top of the table if they beat West Ham in their game in hand on Wednesday, with only two rounds of the season left after that.