The 19-year-old will join Brighton for the rest of the 2016-17 season, with the Seagulls currently just one point behind Championship leaders Newcastle United.
Tomori won Chelsea's Academy Player of the Year last season after helping the Blues retain both the UEFA Youth League and the FA Youth Cup.
The defender can play at centre-back or right-back, and made his full debut for Chelsea on the last day of the 2015-16 season, coming on for Branislav Ivanovic for the last 30 minutes against Leicester City.
Brighton manager Chris Hughton is looking forward to seeing his new recruit in action, stating that he will help provide depth at Falmer Stadium.
"We have brought in Fikayo on loan from Chelsea to provide us with some additional cover in the full-back positions, and we are delighted to welcome him to the club," Hughton told the club's official website.
"He is a highly-rated young full-back, by both Chelsea and England; he is predominantly a right-back, but he can play in any position across the back four, and will give us good defensive cover for the first-team for the second half of the season. "