The 21-year-old, who has been at Chelsea since the age of seven, returned to Stamford Bridge this season following a series of loan spells away.
He spent last season on loan at Derby County under the tutelage of Frank Lampard, and the pair reunited at the start of this campaign at the Bridge.
The departure of David Luiz and Antonio Rudiger’s fitness issues has seen Lampard thrust Tomori into first team action, which has seen him make 16 appearances in all competitions for the Blues.
Tomori scored his first Chelsea goal in the 5-2 win at Wolverhampton Wanderers in September.
Tomori told the club's website: "The club has been so good to me, looking after and developing me during that time into the player and the person I am today. It's a dream come true to sign a new five-year contract.
"I'm really happy the club have shown this faith in me and I'm just excited to carry on."
Chelsea director Marina Granovskaia added: "We are very happy to see another fantastic young player come through from our Academy into the senior team.
"Fikayo has been with us since he was seven years old and has contributed to many Chelsea successes at youth level.
"Having enjoyed an impressive loan spell at Derby last season, Fikayo had a strong start to this campaign and we are enjoying seeing him grow into a key player for us. We are delighted he has committed to the club for the coming years."
Tomori’s performances for Chelsea saw him earn his first cap for England against Kosovo in the EURO 2020 qualifiers last month.
