The 22-year-old will spend the 2014/15 season in the Bundesliga after playing in Spain last term for Valencia.
A statement on chelseafc.com 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."