The 20-year-old Canadian centre-back comes in as Frank Lampard's sixth signing of the summer and joins Blues teammate Mason Mount at the Championship side, having also signed a new deal with his parent club on Monday.
"Derby County have completed a deal to bring highly-rated Chelsea defender Fikayo Tomori to Pride Park Stadium on a season-long loan," the Rams announced via their official website.
Lampard, who has used his great relationship with Chelsea to acquire two of their best young talents on loan deals this summer, told the club's website that Tomori fits the profile and has earned good experience from previous loans at Brighton and Hull City.
“We’re really pleased to bring Fikayo in to provide us with another defensive option in central defence because, over the course of the season, we will need to be adaptable and have competition all over the pitch," the new Derby boss said.
“He fits the profile for us; he is a talented young player and he has already got experience in the Championship from his loans at Brighton and Hull.
“I know him from my connections at Chelsea and, like with the other players we have brought in on loan this summer, I am confident we can be good for them and they can be good for us."