Romeu, 22, has featured frequently in pre-season but will spend another season away from Stamford Bridge.
The Spanish midfielder - who has 33 appearances for the Blues - spent last season with Valencia in La Liga.
Romeu only recently signed a new three-year contract at Stamford Bridge and was thought to be back in Jose Mourinho's plans.
But he will now look to prove himself again - this time in the Bundesliga.
A statement on Chelsea's website read: "Oriol Romeu has today (Monday) joined Stuttgart on a season-long loan.
"The midfielder, 22, who recently signed a three-year contract at Stamford Bridge, heads to the German top flight.
"The Spaniard moved to Stamford Bridge in the summer of 2011 from Barcelona, and has so far made 33 Chelsea appearances. He spent last season on loan at Valencia."
- Sports & Recreation