The versatile 23-year-old defender joins in time for the season opener against Bristol City at the weekend.
Chairman Omar Al Hasawi told the club's official website: "Sam is held in the highest regard by Chelsea and we're grateful to them for allowing him to join us on loan.
"He is an exciting young player who has already shown a lot of grit and determination in his career, characteristics that we are sure will endear him to our supporters."