Despite only joining the Italian giants in January, the midfielder joins Chelsea for the remainder of the 2022/23 campaign, with an option of a permanent transfer at the end of the season.
The 25-year-old defensive midfielder, who made his name in the Bundesliga with Borussia Monchengladbach, only made 11 Serie A appearances for Juve in his short spell at the club.
Zakaria said: "I want to say hi to all the Chelsea fans. I'm very happy and proud to be a Blue and I cannot wait to see you soon at Stamford Bridge."
Chelsea also signed former Arsenal striker Pierre-Emerick Aubameyang and made failed attempts to sign Ajax's Edson Álvarez, PSV Eindhoven's Ibrahim Sangaré and Arsen Zakharyan of Dynamo Moscow.
Meanwhile, left-back Marcos Alonso, who made 212 appearances for Chelsea across six seasons, left the club by mutual consent and is expected to sign a three-year contract at Barcelona.
Chelsea also sent Ethan Ampadu to Spezia on loan, Kenedy to Real Valladolid and sold Billy Gilmour to Brighton for around £10m.