The midfielder leaves Leicester having won their Players' Player and Supporters' Player of the Year awards.
"Determination and motivation – not taking no for an answer," Dewsbury-Hall told chelseafc.com of his career approach.
"One of the most important things as a footballer is self-belief and if you don’t have that then you are going to struggle. I have that and I believe I should be at a football club like this and that is why I am so excited to get going.
"A combination of all of those things put together is obviously a good thing. There are more aspects I would like to add but I am really happy, I am really motivated and determined, and I can’t wait to get going for Chelsea.
"Ever since I was a young kid, I have been one of the most competitive guys in the world.
"I could be playing tiddlywinks and if I lost, you wouldn’t hear from me for the rest of the day. It is part of my DNA. It will never change, and it is partly what has helped me get to the level I am at now.
"I am always motivated and trying to do my best and trying to win. Anything else, I look at as a failure. It has stood me in good stead so far and hopefully it continues to help me and help this football club."