Kerr will be available for Emma Hayes' league leaders from January after securing a third consecutive Golden Boot in the NWSL with Chicago Red Stars.
The 26-year-old netted 19 goals in 23 games to be named the competition's MVP, although the Red Stars missed out on glory after suffering a defeat to North Carolina Courage.
Kerr swept the board in terms of individual awards despite taking a mid-season break to feature at the World Cup in France, where she scored five goals for Australia.
She has 38 goals overall for the Matildas over the course of an international career that began at the age of 15, and she is on the shortlist for the Ballon d'Or Feminin after also receiving a nomination at the FIFA Best Awards.
Kerr told Chelsea's official website: "The WSL is the best league in Europe. I want team success and I don't want it to come easy.
"I feel Chelsea have been building something special over the years and I want to be a part of that – I want to lift some trophies."
Chelsea manager Hayes said: ‘Sam has proved time and time again in the NWSL that she is a prolific goalscorer. She's won the Golden Boot many times and she’s a player that can make things happen, but she’s also a fantastic team player.
"The fact she chose Chelsea when she could have gone to any club in the world is a testament to the players and the staff here because she saw this is the best place for her to grow and take the next step in her career. That's a wonderful compliment to us all."
Chelsea has a one-point advantage over Arsenal and Manchester City at the WSL summit having won four times and drawn once amid an unbeaten start to the campaign.