Derby have now revealed that Tomori has followed the young playmaker to Pride Park, with the 20-year-old also due to spend the campaign with the East Midlands outfit.
Lampard told the club's official website: "We're really pleased to bring Fikayo in to provide us with another defensive option. 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."
The versatile Englishman has already made a total of 34 appearances at the second tier.