The 19-year-old, who was born in Canada, made his Blues debut on the final day of last season against Premier League champions Leicester.
Tomori has signed in time to be involved in the Seagulls' Championship game at home to Cardiff on Tuesday.
"He'll give us good defensive cover for the first-team for the second half of the season," said boss Chris Hughton.
Brighton are second in the table but will go above leaders Newcastle if they win their game in hand against the Bluebirds.
Find all the latest football transfers on our dedicated page or visit our Premier League tracker here.
This site is optimised for modern web browsers, and does not fully support your browser
Brighton have signed Chelsea and England Under-20 defender Fikayo Tomori on loan until the end of the season.