After securing deals for fellow Blues teammate Mason Mount, Liverpool highly-rated winger Harry Wilson, Florian Jozefzoon, Jack Marriott and George Evans, Frank Lampard welcomed the versatile defender to the Pride Park Stadium this summer.
This deal represents the third time Tomori would be allowed to leave Stamford Bridge on a temporary, with the 2016-17 campaign spent at Hull City.
With the Tigers, he played 25 times in the English Championship, and Lampard is delighted with the signing of a new centre-back option.
"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," Lampard told club website.
“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."