The 23-year-old central defender moves to the Madejski Stadium having spent the first half of the campaign in the French top flight with Nantes, where he made eight league appearances.
A USA international, Miazga signed for Chelsea in January 2016 from New York Red Bulls, where he had progressed through the ranks before making his debut at the age of 18.
Composed on the ball and a good reader of the game, Miazga made his first appearance for the Blues when he started in a 4-0 Premier League win away at Aston Villa, partnering Branislav Ivanovic at the heart of the defence and performing well as we kept a clean sheet.
He was handed another start a week later, alongside Ivanovic once again, although on that occasion he was part of a Chelsea side beaten 1-0 away at Swansea City.
Miazga spent the 2016/17 season on loan at Vitesse Arnhem and he enjoyed a strong finish to the campaign as the team won the Dutch Cup final, the first major trophy triumph in their history.
Good luck at Reading, @mattmiazga3! ?
— Chelsea FC (@ChelseaFC) January 25, 2019
After returning to Vitesse on loan last season, he established himself as an important member of the side and made a total of 42 appearances, scoring four goals in the process.
Miazga has earned senior recognition for USA and during the October international break he started consecutive matches against both Colombia and England.
He will now continue his development in the Championship with Reading, who are currently 22nd in the table.