The 27-year-old centre-back has agreed a five-year contract with the Italian side, who secured Champions League football last season after finishing fourth in Serie A.
Chelsea will receive an initial 25.7 million, with a further 5.1 million available through performance-related add-ons. The agreement was finalised following lengthy negotiations between the two clubs.
"Trevoh is an important signing for us and a player who represents the level of ambition we have at this club," said Fabregas.
"He is a leader who can raise the level of the team, and I am very happy that he has chosen Como."
Chalobah leaves his boyhood club after almost 20 years with Chelsea. He joined the academy as a child and made 151 appearances for the first team in all competitions, winning five trophies during his time at Stamford Bridge.
His honours included the FA Cup in 2018 and the Conference League in 2025. He also spent time away from Chelsea on loan at Ipswich Town, Huddersfield Town, Lorient and Crystal Palace.
"I'm thrilled and honored to be here at Como: it's an ambitious, rising project, and working with manager Cesc Fabregas again is truly special," said Chalobah.
Chalobah has earned two senior England caps and was a late call-up to the Three Lions' World Cup squad.
