Gilchrist has been involved for the club in the FA Cup this season, but has not featured in the league.
The 20-year-old is now tied to the club until 2026, with the option of a further year.
The defender, who can play centre half and right-back, is highly rated at Stamford Bridge.
He told the club's official website in January: My grandad and my dad used to have season tickets. They used to go all the time, and my aunty used to go with them as well sometimes. So I obviously support Chelsea as well.
I remember watching the Champions League final the first time we won it in my uncle's living room. That was a good memory.
Actually, my dad had a Football Manager game, or whatever it was called back then, and before I was even born, he made a player called 'Alfie Gilchrist', who played for Chelsea. He had chosen the name and everything, and he played with that character as me at Chelsea. So it's crazy how it's come to life."