Romeu left Barcelona for Stamford Bridge in 2011 but spent last season back in Spain on loan at Valencia.
The 22-year-old signed a new three-year contract in July and has appeared in all of Chelsea's pre-season fixtures, with the exception of Sunday's 3-0 defeat to Werder Bremen.
A statement on the club's official website said: "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."
Romeu becomes the fifth player to be loaned out by Chelsea this summer following Tomas Kalas, Mario Pasalic, Lucas Piazon and Ryan Bertrand.