The 20-year-old, who stands at 6'4", had been sought by a number of Premier League clubs this window after impressing in the MLS.
"I am very happy to sign for Chelsea, a world-famous club with a reputation for major success," said Miazga. "I am delighted to be in the Premier League, the best league in the world, and can't wait to meet my teammates and get going.
"I am still at the beginning of my career but I believe this is the place to develop and learn from some of the best players in the world."
Miazga earned his first international cap for the USA last November.