The 22-year-old has made 10 appearances so far in his debut season for the Blues, scoring twice in the Premier League.
Chalobah is a graduate from the Chelsea academy system and has previously spent time on loan at Ipswich Town, Huddersfield and French side Lorient before getting a breakthrough at Stamford Bridge.
"It's quite surreal," Chalobah, who joined Chelsea aged eight, told the club website. "It's a dream for me to sign a new long-term contract for my boyhood club and hopefully the dream continues.
"It's been an unbelievable start to the season and something I never expected but it just shows that the hard work I've put in and going out on loans had paid off.
"I tried to impress the manager in pre-season and then take that into the games so it's been good and consistency now is the key to keep those high levels."
Chelsea director Marina Granovskaia added: "To see Trevoh joining so many of his fellow Academy graduates in the men's first team is something we are exceptionally proud of as a Club. When he scored in the first game of the Premier League season, it was an emotional moment for everyone involved in his progress to this point.
"His performances in the early months of the season, as well as his attitude and professionalism, have impressed us all and we are very pleased to be able to reward Trevoh's progress with this new long-term contract."
